I would go with pulling it off and checking the key it won't take much for the key to jam and prevent it from going in also what you can do that i do is take a long framers square and place it on the balancer flush with the face and see just how far it is out you might just find you have a worn pulley that the belt is sitting to far down. Take a spare belt and flip it inside out and check your pulleys there should be a small space between them and the belt