Sounds like you have one of the propellers with a spun hub, or split in the blade. Does it flip over the same direction every time? Possible that the servo is stuck to the far, left, or far right?
If your not getting lift on one side put your hand under each blade and feel for equal air pressure from each blade. Also possibly check your blade angles, you could have a blade out of whack or bent. try flipping the propellers from left to right side and see if you get the opposite results that will narrow it done to your propeller/s lift is produced by blade angle & speed, it might be one blade needs a micro adjustment.