if you've got several hours on the unit, or you notice that the tail motor is getting warm, it just might need a new motor.

Some other things you could look at are is there a short or a loose connection or corrosion on some contact to the tail wire? Are you getting good signal? Ie the receiver wire in tact, the Tx has good batteries, the antenna is routed away from the motors ect.

Most likely though the motor is failing and needs to be replaced.