Yesterday I tried to get my holding tank pumped out but the dockhand at the marina couldn't get it to create any suction. He said there should be a valve to turn it between the marcerator and the pump out outlet, but there doesn't appear to be one. There is the pump on the top of the holding tank and it has two hoses coming off of it, one on either side. One hose goes to the thru-hull fitting for overboard discharge, and the other goes straight up to the pump out fitting on the the deck. What could be wrong? Why couldn't the pump out create suction?