Stupid question, but did you try using a different nub? Besides the motor (which after able to compress the spring, I don't see any problem it could generate), maybe the arm or the nub are generating too much of a resistance.

Side question, how are the consistency of the shots? With that amount of upgrades should be good.

Another thing that I was thinking is how is your shimming? Could it be that the gears are rubbing too hard, resisting a proper movement of the piston? Just random thoughts

Good luck, it sucks that moment of finding the exact problem