I moved into a house that has a utility sink in the basement. A pump is installed beneath the sink to pump the water to the waste line. Anyway, the pump has a manual switch to activate. When I flip the switch the pump makes a low hum, but doesn't move any water. It's a "Shellback" brand pump and says it has only one moving part. Can anyone suggest why this is not working and if it may be an easy item to repair? Thanks.