So even though the new buckings haven't arrived yet, after taking it all apart and reassembling it I noticed that the range of motion of the gear was much larger than it was before, and indeed I was able to get some visible movement out of the bucking. I reinstalled it in the gun and took it out to test and it's working great now! I'll probably still put a new bucking in when they arrive, but it seems the problem has been largely solved. I guess something about the gears wasn't quite aligned/right and it was catching and preventing the hopup from engaging fully.
In any case thanks for the help guys - everything with the gun is working great now
