DO NOT FLY IT!!!
Either there is something inside the servo preventing the gears from turning, most likely a broken tooth, or the servo is just on its way out. I have had two of the stock servos stop working on me. The first time one failed I just happened to notice right before taking off that the elevator servo wasn't moving. The second time it happened a few seconds after take off and went side ways right into the ground.
It was the left servo that must have stopped and as I added collective to ascend it caused it to roll.
I would not fly with a servo that is working intermediately. That is just playing with fire. Either upgrade the servos or just send it in to Horizon Hobby and they will replace it free of charge.