Firing in full auto when in semi

[1Hunter1]

Hi all, My M4 keeps firing in full auto when I put it in semi and I have no idea why. So far I have tried 2 different selector plates, 2 different gearbox shells, 2 different cogs ( the ones that the tappet plate goes on to), 2 different cut off leavers and I fitted a new trigger set with the wiring. As far as I know I have covered everything but still got the same thing going on. Any help with this would be great thanks