which cassette sprocket is the problem one? that might help with the diagnosis.

I have a feeling that someone is going to suggest that you check how straight your rear derailleur hanger is.

I was going to but resisted.... ;-)

I suspect the issue might be a very slightly worn / kinked cable or housing. The cable is having a little trouble getting past one spot.

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Newbie-but-learning husband-mechanic here. Here's a list of things that I've had to do to get a bike to shift well in all the gears, when it seems to be shifting and running quiet in most of the gears. Some is tongue in cheek, some might be helpful, your guess is as good as mine as to which is which because I'm lost half the time I'm fighting with my wife's bike.

Replace cable.
Derailleur high and low limit.
Turn the barrel adjuster one click one way or the other.
Re-tighten the cable, especially if you (or I) installed it. I use pliers to pull tight while I turn screw to clamp it to rear mech. Repeat, after you do it wrong the first 3 times.
Turn the barrel adjuster a few clicks to re-index.

Yup still makes noise in that one gear.

Take wheel off, get derailleur hanger tool out. (expensive! also worth it) Straighten derailleur hanger.

Remember B screw. Play with that a bit. Set it so derailleur is as close as possible to cogs, but not so close it makes ugly noise when shifting into largest (easiest) cog.

Still makes noise.

Adjust B screw out just a bit. Twist barrel adjuster exactly one click one way or the other!

Bam, perfect shifting. That was earlier this week ;)

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Sounds like a classic case of the cable naturally stretching, and right on the verge of becoming an issue, with more than just one gear. Try a few clicks CCW on the adjustment barrel, and I bet it's good as new. That's why the adjusters are there. Cable stretch is normal. Eventually shifting is effected, and it has to start somewhere.

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Athlinks / Strava

Guys/Gals, first off thank you very much, a lot of good comments and suggestions here. Having said that, I've tried 95% of them, to include checking the Rear D hanger alignment (dealt with that insidious problem before, not fun).

The gear in question is the seventh from bottom, fifth from the top (#17T) - apologies should have included that in the original post. As the cassette is an 11-25, it's also not associated with the jump (i.e. 17 to 19), it just doesn't like to shift from 16 to 17. (The gearing runs 11-17, 19, 21, 23, 25)

My suspicion is it's the gear itself, maybe there's a mar on the step of the gear that isn't engaging the chain correctly.

My other theory is much like many of you already mentioned, an issue with the cable.

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"It's easy to be hard, it's hard to be smart"

1st question: does it happen with both chainrings (unless you ride 1x)?
2nd: do you have other cassettes in your disposition to try (spare wheel, direct drive trainer etc.)? if not, you should borrow one and verify if it works fine: if it does, then your 17t cog is shot, otherwise, the problem is somewhere else in the drive train. please note that with the different cassette, the issue might happen on a different cog, possibly still around position 6-7-8

This sounds ridiculous (because it is) but I had this same symptom once because when I was assembling the cassette I put one of the cogs on backwards. (I blame a late night in the garage, rushing to get the bike back together before a ride). Because the teeth on the Shimano cassette are angled a little bit, it behaved the same way in that one gear. I had to shift past it and then retreat to get into that gear, but only for that one gear. After looking at everything else, I eventually got close to the cassette and realized what I'd done wrong. Once I got all the cogs pointing the correct way, it was a much nicer riding experience.

As far as I know, it isn't possible to put a Shimano cassette cog on backwards.

I'd put it differently - the cable will stretch (really housing/etc bedding in) in the first few minutes or hours of use. The adjusters are of us in initial set up and those early changes. If it starts stretching after that, something is wrong.

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I have had a very similar problem and it was a frayed shifter cable. The shift would not happen right at the exact gear because that is where the fray ran into the resistance. In the drop back it slid better and dropped in. I suspect the fraying is close to the shifter. I know how frustrating this one is to solve.

That's the very first thing I thought of as well.

Cable hang up.
Shifts good one way but not the other.
Make adjustment to correct the bad way and the problem reverses.

I wouldn't think hanger alignment because the problem is in the middle. Hanger alignment is normally a problem at one extreme end of the cassette.

Cables continue to stretch for several months of continuous use. It’s one of things we serviced, and one of the reasons bike manufacturers offer free 6 month checks ups, on new bikes. I did hundreds of those check ups, and part of it was to zero the adjusters, and reset the cables tight. They will still stretch over time, but much less... maybe 3-4 clicks on the adjuster, over the cables lifetime. Again, that’s why the adjusters are there.

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Athlinks / Strava

Again, thank you all for the terrific feedback and insight - it is priceless knowledge.

Concur, Rear D hanger alignment is ruled out because of gear location (again, should have included that in my original post).
The cable hanging up at that point makes sense, will investigate.

Next steps are going to be:
1. Check the gear itself to make sure it's not inverted, or at the least see if there's any noticeable deviation/damage to that gear from the others.
2. Replace the cable and clean up the Rear D (I'm kinda OCD when it comes to drive train cleanliness but still...)

MTF...

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"It's easy to be hard, it's hard to be smart"

Exactly. Just as a chain stretches throughout its life, the cables do the same.

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"It's easy to be hard, it's hard to be smart"

It only happens in the big ring. If I'm in the small ring it shifts fine. The cassette in question is the one I have on my indoor trainer (Wahoo Kickr), I have an identical cassette on my rear wheel that does not experience the same problem (at least I don't think it does, it's been a bit since I've ridden outdoors).

For the reasons stated, I'm leaning toward it being a single gear issue. However, the cable is also a suspect.

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"It's easy to be hard, it's hard to be smart"

Chains "stretch" because friction increases the spaces between parts. This happens throughout the life of the chain. The mechanism is totally different than cable stretch, which is not from friction but the cable being pulled taut and the housing and other parts bedding in; these are short processes

My cables don't stretch after the first several hours of use. The only times they have, I noticed they were fraying or there was another problem. This is with excellent housing cut well, using ferrules when possible. Perhaps my installations are at a higher level or I'm somehow lucky. Or maybe my shitfing is poor and I don't notice it. I don't.

They're also there for precise adjustment on setup or for little tweaking with different spacing/location of cogs relative to the derailleur.

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If you have a quick connect link on your chain, take a peek at it to make sure it’s fastened all the way. Just another small step to look at.

Cables do not stretch at all.
What happens is that the casing beds into the ferrules and also settles itself if the ends have not been cut square.
This mainly happens if the section has a bend in it like the loop near the rear derailleur.
You can minimise this with good installation.
There should not however be any major change throughout the useful life of the cable after initial settling.
Usually there is no change and a hanger is bent if there is issues.

The OP has stated that it only happens in one front ring.
This rules out a cable problem unless the angle of the derailleur change when using different rings is pulling on the cable.
If the shifting changes from one ring to another the problem is the hanger or the parallel-o-gram of the derailleur is bent.
If the chain is too short it may also be pulling too far from the cassette in the big ring and exasperating any cable friction problems.
Align the hanger.
Check derailleur for damage.
Remove friction from the cable.
Consider chain length problem.
Check pulleys for free spinning and free float on top pulley.
Check housing length is adequate.
Check shifter by using another on a short length of cable attached direct to derailleur, this isolates the entire shifter/cable system in one go and will tell you which side of the equation to continue looking at.

I'm not going to argue with someone I don't know, about cable stretching. But I will speak up when something isn't true. All cables stretch. Automotive parking brake cables stretch. Aircraft control cables stretch. Guitar strings stretch. Weight machine cables stretch. And you better believe that bike cables stretch. And the list is endless. One thing all these things have in common, is they all have a way to adjust the cables for stretch, over their life. Adjusters, tuners, turnbuckles, whatever. I've been a mechanic my whole life, and have adjusted cables on many things, in many different ways, and especially on my bikes and friends bikes, and customers bikes when I worked at a bike shop. I've built hundreds of new bikes, with factory cut casings. It was standard practice to stretch the cables on initial build up. Then check the showroom floor bikes regularly, and do a final check when a bike sold, before it went out the door. We also stressed how important it was to return in 3-6 months for the free purchase check up, and resetting the cables was an important part of that. And I couldn't count the hundreds of times customers would come by the shop with late model bikes, freaking out because the gears were starting to chatter, or they let them get to the point of a gear or two not shifting, or the classic shifting past a gear, and then coming back down to the gear they want. A quick glance to make sure everything was ok, screw the adjuster out a few clicks, tell them to try it out in the parking lot, and send then on their way, thinking you were a miracle worker. Cables stretch a LOT when brand new... even on old bikes with down tube shifters, and little or ZERO casing involved, as is the case with many older front derailleurs. After the initial stretch, they continue to stretch much less, for several months, but enough to need small adjustment, but are pretty stable after about 6 months of use. After that they may go a year or two, until you need to adjust anything.

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Athlinks / Strava

Current bike cables come pre-stretched so the strands are aligned and as straight as they are ever going to be.
The examples you gave all have hundreds of kilos load on them, bicycles very much less.
The examples you quoted all have multi bundle cables which due to their construction will always settle during life.
Most of the so called stretch even in these setups occur at the cable terminations, usually a thimble and clamp arrangement.
Bicycle cables are a single wound bundle.
Most of the outer shrinkage you believe is cable stretch occurs in the rear loop where the cable casing tries to unwind and equalise the strands lengths as it is usually cut straight and then bent to fit the bike.
It is there in a semi constrained state and will change throughout its life when fitted this way.
If you bend the casing and then cut it this does not happen and the cable setting will remain constant throughout the life of the cable and casing.
Even down tube shifting bikes have this loop.

Poorly done cable jobs also never get grease under the endcaps when fitting, this allows the cable ends to rust which then causes cable drag and casing loss at the cable casing end.
All of this adds up to what looks like cable stretch, but single wound cables of that dimension under typical bicycle loads do not stretch as testified by the many good cabling jobs that do not require any adjustment though-out the life of the casing which ultimately comes to an end when the housing outer cracks and lets rust form or the cable breaks from fatigue.
Most sensible people will change the cable before then though.
Not one of my bikes has ever required an in use adjustment of the cable, nor have my customers apart from a temporary adjustment because of a bent hanger.
Brake cables are under huge loads compared to gear cables and only require adjustment because of pad wear, not cable stretch.

I’m certainly not going to argue, as I’ve been doing this my whole life. I’m also positive, that we will never change our minds. But I am impressed at your knowledge, and ability to write. I did have to LOL today and thought about this thread. I overhauled my 34 year old bike over the winter, and put new shifter cables on it, pre stretched them and all. It has downtube shifters, and absolutely zero outer casing on the front derailleur. It routes under the frame, and is 100% exposed. The shifter and derailleur are 34 years old and have been settled for over 30 years. On our ride today, the cable was doing it’s normal 3 month initial sag. I stopped, and took about 1-2 mm stretch out of it. Now it’s good to go for probably a year or so. The rear derailleur took 3 clicks, to dial in perfect. In a few months, I’ll zero the adjuster, and reset the cable, and it will be good for a long time. The casing loop is the only outer casing in that system, and could be as much as 30+ years old, I have no idea, as I don’t change casing if it’s still good, and then there no fussing with compression. The brake casing on that bike is still original. I grease all my cable casing with high pro automotive grease. Don’t even get me started on my 55 year old bike. I love old stuff, and keeping it going forever.

All one has to do is google “ bicycle cable stretch” and you will immediately see that this debate has been going on forever, and will probably keep going on until the end of time.

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Athlinks / Strava

Replace that cable! I will guess that it has started to fray right where the cable leaves the shifter and enters the housing. I've seen this a bunch of times with virtually all shifters but some more than others (7800 Dura Ace but also 9001 and Campy Chorus from a few generations ago). The act of shifting pulls or releases the cable around a spool in the shifter body, after many thousands of bends/unbends around the spool the individual wires in the cable become work hardened and break. At first it is usually one or two strands that cause the entire cable to hang a little but only under certain conditions (probably particular to a bike due to what the most common shift points are for a give rider) and this is when the shifting gymnastics start to show up, i.e. overshifting to complete a shift but only in one gear.

Even if it is not this, or the kinked cable in the rear housing mentioned above, replacing the cable (and maybe the housing too) is probably a good and cheap place to start.

Rich