Something might be wrong with your servo that moves the blades up and down. Check to see if it moves up. Something might be wrong inside your radio too. I had that problem. Try cleaning your radio wires that control the servos. It might be a loose wire in the radio. Your blades hit againsts something slowing them down? Check that too. If you fly too much with that problem the main motor might burn down because of too much force! Check the main drive gear and make sure that all the little "Teeth" on the gear are even. It might be cracked.