actually what you describe sounds like the plastic arm is broken, its kind of a two piece arm that has a part connected to the motor that moves when you operate the remote, and outside of that is the arm that moves the knob. so if the outer arm isnt broken the knob will move like its locking and unlocking, but the inner arm is broken and not actually moving the lock mechanism in the latch.you have to replace the whole mechanism as it comes as one unit.