Adjust your motor using the adjustment screw (put some loctite on the screw before) located below your grip until the motor turn the gears...
http://www.loctite.com/int_henkel/lo...d=114&layout=3
check for assembly product---threadlocker---removable strenght----threadlocker small screw : product is 222
My M4 was doing that from time to time... The motor adjustment screw get loose from the vibration