cable could be slightly too tight -try slackening off very slightly.

when you shift up you tighten the cable so if it's slightly tight already it will always shift up ok but when you shift down (and slacken the cable) if may not go straight away as it's slightly too tight to start with.

just slacken a small bit to start with and usually this sorts it.

Since cables housings shorten with age I would expect most systems to shift down better than up without any adjustments.

Yours is going the other way which suggests that the cable in hanging up somewhere, most likely in the housing.

Replace the cable and housing.

Look at the spring. I bet it is stretched or broken.

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"...the street finds its own uses for things"

It's a new mech

Verify the the cable routing is correct at the RD cable locking bolt. The light action SRAM derailleurs seem to be very sensitive to this routing.

It has to be cable sticking in the housing, or weak spring on the derailleur. Disconnect the cable, push the derailleur by hand and see if it's returning okay. Then check to see how free the cable moves inside the housing while disconnected.

Thanks chaps, I think a recable is in order.

If the RD cable is exposed beneath the bottom bracket, there is a chance you have some dried up sports drink gunking it up. Try cleaning that area with something liek windex or wd-40 and making sure there is nothing sticky that could be causing the cable to drag.