I have a 4 year old Doheny's Discovery that will not move when I turn it on. More specifically, it will not perform the normal self-test cycle at the beginning of its operation. When I turn on the unit, it moves about a foot in reverse, but it will not move the same foot forward in order to complete the self-test cycle. It then seems to shut itself off. I have taken it apart to make sure there are no pieces of palm tree or anything else that would tangle the impeller, wheels, etc. Any ideas as to what might be the cause of this problem would be appreciated. Thank you!