Try to change the coin cell battery in the speed/cadence sensor. A low battery could create connection issues. How far is the ant+/bluetooth sensor? I use to get dropouts and I purchased an active USB wire extender to bring the ant+ sensor within 1 feet of my trainer.

Also make sure that the trainer is calibrated properly for each bike. If the pressure on the back wheel is too loose/tight the power output will be completely wrong.

I hope that helps!

Thanks Charles. I managed to notice that my wheel size was set to 20inch wheels when i was on which is probably why my speed was incorrect.

So, now i'm just left with one problem. Why would the signal be ok for my bike but become intermittent when my son is on his bike? We're using the exact same sensors.

If your speed/cadence sensor is the Garmin combo unit that zip ties to the non-drive side chain stay and reads a spoke-mounted magnet...is it possible that the spoke-mounted magnet or speed sensor arm are not positioned properly? Maybe it is reading the magnet intermittently (sometimes normally and then sometimes only once for every two or three revolutions of the wheel). That could result in fluctuations in displayed watts of between 100%, 50%, or 33%. The Garmin version of the speed/ cadence combo sensor has a LED that blinks each time the magnet triggers the sensor. You should be able to use that LED to confirm the magnet and/or sensor placement while spinning the wheel. I would spin the wheel up to speed but then stop the pedals/crank so the cadence magnet on the end of the crank arm doesn't trigger the LED and confuse things.

My child uses Zwift with the above setup and with a small road bike, and the placement of the sensors and magnets is finicky.

If you are using a hub-mounted Garmin speed sensor or some other brand, then I have no ideas for you try. Best of luck to you and your kid.

Are you simultaneously sending data from the speed and cadence sensors to a bike computer or watch using the same method (ANT+?)?
If so, does that device record the same intermittent data as Zwift or are they receiving data consistently?
If there is a difference between the two, it seems unlikely the sensors are at fault, and I would suspect something is causing communication problems when your son is using the system. My guess would be it's the different location of the sensors relative to the ANT+ dongle on your Zwift device. Could perhaps also be other devices in the are interfering with the signal that he uses and you don't or the like.....