The middle arm of my dishwasher is not spinning and my dishes are not getting clean. I have checked both arms for leaks or splits and they are fine. I have checked the pump and it has plenty of pressure and it is fine. I have checked the check valve and everything I can think of. There is a ball in there the size of a marble that is supposed to move up and then back down, I think, but I don't reallly see how that works. Any ideas on what is wrong? Thanks