Thanks for the info... took the gun apart completely to have a good look into it.
Turns out the teeth on the piston had been shredded to the point where it wasn't pushing back enough to make contact with the recoil piston. Would still fire (at like 200fps less than usual lol) and after grabbing a chrono i did some testing. Replaced the piston, did some TLC to the internals (mainly consisting of cleaning out all the plastic shavings which were scattered through the gearbox 😬) - all working now. Thanks for the info anyway!