one of my 6101s has stopped working. when it tries to start, the impeller makes 1/2 turns sporadically for about 10 secs and then stops altogether. I have tried it on both of my power supplies (other pump is working fine) with no luck. Is this a known problem? There is no calcium buildup and the impeller and housing are in good shape. Pump is only about a year old. What do I need to do to repair?