Most likely the gear is slipping on the motor shaft or on the rotor blade shaft. The rotor shaft gear might have a very small set screw that can be tightened. If there is no set screw and it is slipping then is probably cracked, but the crack will be so small you can't see it. Everything is very small and difficult to see on that helicopter, you will probably need a magnifying glass to see the motor gear at all. You would probably be better to get a new heli and use that one for parts!