The SeaLand T-series discharge pump (it isn't a macerator) is basically an electrified version of a manual diaphragm bilge pump. The most common reason it won't pump is that one (or both) duckbills need to be replaced. It is fairly easy to do, albeit odiferous (stinky), and that should be the first thing to check after making certain that the holding tank's vent line isn't plugged-up. The duckbill valves come in sets of two, and both should be replaced at the same time. One on the suction side, one on the discharge side.