I had the same problem, Once when I pluged in the battery I heard a kind on hum from one of the servos then it stoped, I thought nothing of it until I spooled up the main rotor and couldn't get off the ground. I looked everything over and come to find out that the front servo had a broken gear in it. You can look through the blue case and see the gears on top of it and I noticed one was out of place I tried to fix it but the little stud the gear went on was broken off from the case. I called Tower Hobbies and they gave me the number to the helimax warranty people, gave them a quick call and they sent me out a new servo. Everthing work great now. I hope this helps with your problem.

JP