Generally speaking it's gonna be a problem with the selector plate or the switch itself.
I had an mp5 do this on me when I swapped a g&g selector plate on a CA one, and did some fiddling around with the selector switch itself.
Save yourself the headache; get OEM parts if you can. The only difference between a g36 and an ak gearbox is the nozzle length so even if you swapped it with a g36 selector plate it should still work. Only thing now is that the selector switch or the selector plate may be off spec or wrong brand so it's not on the right firing mode when you switch.
I make replacement polycarbonate flashlight lenses! Send me a PM.