Seems you've solved the problem, but it may also be caused by the sector gear not holding the tappet plate back long enough for a BB to feed every time.
My mate has a G&G GR16, which used to misfeed every other shot on auto. Spoke to Fire Support, they said it's a common problem in the G&Gs.
Said to get a
delayer for the sector gear. It clips onto the sector gear (hole in it goes over the bit that sticks out, and the little indent in it sits against the centre of the gear to stop the delayer rotating).
It requires quite a lot of force to put on, but it's definitely not going to come off in use.
Now no misfeed problems at all.