SHS V3 Tappet Plate
The tappet plate is a plastic part in the gearbox that houses the air nozzle and is responsible for retracting it to the rear allowing a BB to enter the chamber. This movement is achieved thanks to a metal cam on the sector gear which pulls the tappet plate back, which in turn moves the nozzle. Now since you've got metal on plastic contact, you can expect wear and tear. Eventually this will mean that the nozzle won't be retracted enough for a BB to pass. The only way to fix this is to replace the tappet plate.
This particular tappet plate is for V3 gearboxes and will do AK-series guns (AK47, AK74, AKM, AIMS etc), G36, MP5K, AUG, Galil, SIG etc.
Consider the addition of a sector chip whilst you have the gearbox open, as this will increase the amount of time the nozzle is retracted enabling smoother feeding.