Ande

The motor in my m4 started whizzing so i took it apart, there was some wear on the piston and the gear on the motor looked a bit rounded so i just fitted a new piston, new motor, shimmed it up, put it back together and it still whizzes, i put the motor in and tightened the cap back down on the grip, when i pulled the trigger nothing happened except for the wires getting hot, so i unscrewed the cap on the grip and it started to move, but only whizzing, if i try and press the motor onto the other gear it screeches as it makes contact then as i apply more pressure it just stops or locks up or whatever. I don't know what to try next, is it the gear that makes contact with the motor gear?

Me and my son were out back yesterday target shooting with our M4A1, when it just stopped firing and started this screeching noise, i looked on youtube for some advice and saw somehwere to adjust the allen screw in the bottom of the grip, that just lessened the screeching one way or stopped the gun altogether the other way, so i stripped the gearbox to see if i could find any damage, everything looks fine apart from some wear on the last 2-3 maybe 4 teeth, i have stripped it down and put it back together twice just to double check i had assembled it right, still does it, motor got very hot so i disconnected the battery and put it all back together, any ideas what it could be? it all looks fairly simple inside so i can't see what has happened.