Maybe you can make a video of your issue and post it on on their “support” server on discord. I have seen all world problems getting solved on that server.
The flybywire system is a very deep and fragile system. Anything could be affecting your throttle, could be not enough deadzones, could be an issue with your controller (or not), could be that your throttle axis needs calibration again, could be AT not activated etc etc etc…