[Schwag]

Motor too high probably would not let it turn that much. Shimmed too tight is very possible.
Definitely check your battery.
Assuming battery is ok, make sure you're not shorting anything and you have a good connection where you removed the fuse.

Are there any new parts in the gun? I've had pistons that weren't quite right that forced the teeth to lockup halfway throught the cycle.

Open the box, remove the anti-reverse and tappet spring, then spin it by hand while trying to keep the piston/gears aligned as they would be if the box was closed. Feel for tight spots. Check the teeth in your gears to make sure nothing is binding there.
Make sure your spring can actually compress into your piston.

Good luck!