Hammer's got it right - pretty sure its your selector plate if you didn't open up the gearbox.
Just to clarify, did you take your gearbox out of the gun completely (but didn't open it) or did you take it you just break your gun down to the gearbox in the lower pistol grip/receiver area?
If you pulled your gearbox out of the lower half of the AEG, then you’ve probably misaligned the selector plate.
I’d put the selector switch in semi-automatic, and then drop the gearbox back into the lower receiver.