Its possible fibers from carpet or clothing or hair fibers are wound between prop and motor. See instuctions to remove prop then debris can be easily removed from motor shaft. Do not press prop onto shaft to tightly. If this doesnt work motor might be worn or shot.

Without either gps, optical or pressure sensors stable hovering is not possible. Trim adjustment is used to reduce drift. Look into all the forces acting on these quads and you will see how amazing it is they can even fly! Happy Hovering!