Frigidaire Dishwasher model # fdb635rfr4 top arm...

Question

My dishwasher's top arm will not spin. I have checked the pump and both arms and they are all fine. Plenty of pressure and no leaks or splits. There is a little ball inside one part that is supposed to move up and then down, but I can't figure out how that part works or if it is possibly the problem.Do you have any ideas? Thanks