Downshift protection in-game feature explained
If you occasionally experience an issue where the downshift command is not accepted, this is not necessarily due to a hardware-related issue with your steering wheel or shifter paddles.
In real vehicles and simulations, there is often a "Downshift Protection" that prevents damage to the vehicle when downshifting at high RPM. When downshifting to a lower gear, the RPM increases at the same speed. It is possible that the RPM would be too high after the shift, and therefore, the vehicle (even in the simulation) prevents the downshift.
To check if this might be caused by an issue with the steering wheel:
Try to downshift several times in a row when the car is definitely in a lower rev-range. Check if the downshift works all the time.
Check if the same issue is happening in another game.
Check if the shift inputs from your wheel or shifter are always detected in the Fanatec app on PC.
If you have any other wheel, please compare it and check if the issues are only present on one wheel. Please always keep a possible downshift protection in mind.
A definite check could also be to remap up and down shifts to another button or switch the pedals. Like this you could isolate a hardware issue. If this hops to another physical button too, the hardware should not be the root cause.