More often than not the opposite occurs and you're stuck with no semi auto, which happens because a part wears out, of which I forget the name...
I imagine for you it'll just be something to do with the selector plate, which is external to the gearbox and is what the fire selector lever moves when you turn it, in order to change the fire mode.
There's a similar problem with the M4 I'm selling. The selector plate on mine has slipped slightly, so when you switch to auto it still fires semi for the first shot, then auto afterwards because the selector lever doesn't move the selector plate far enough to properly engage auto.
In short, you need to get the gearbox out and correct the seating position of the selector plate, a spring might've potentially fallen out of it as well. It doesn't require gearbox disassembly, just to take the gearbox out of the gun and all the parts you need to fiddle with are external.
In all likelihood, simply taking the gearbox out, giving it a once over and putting it back in will probably sort it out, you probably won't even need to really do anything.
Ought to be an easy enough fix.
Good luck.
Oh, and Merry Christmas
