[Sgt.Hartman]

hi what make that problem can be:
1- the little spring on the selector plate
2- inside the gearbox, behind the electric assembly for trigger, there is a little moving part, like a rocker. Be sure you didnt forget to put it and it is easy to move, this is what make the gun stop after one shot, the cam on the gear make it move and move the contact pilot to get back to its place and stop the contact. its like a rocker on a fuel motor.

if it was working before, it can only be those 2 possibility.
good luck