P90 not firing properly
 

Has anyone had an issue where a P90 would fire full auto but not in semi. I've tried several batteries and checked out the mechbox. It doesn't seem to be anything noticable. I've never worked with the switch assembly of a P90, only version 2 and 3 mechboxes.

Ideas?

What kind of BB's are you using?

I'm not, I'm just dry firing as a test and when I pull the trigger in semi, nothing happens. Or rather i hear the motor trying to turn but doesn't. It shouldn't be a shimming job because I've looked at the gears and they spin right and it does fire in full auto without any problems.