I had the same problem with our 321, and found two problems, both identified above: I had not set the valves in the correct position to pump from the bilge (from pumping out the icebox)...; and, my strainer had a bad seal, so the pump was just pulling air around the strainer cage. I replaced the 'pump guard' strainer (and set the valves in the correct position...) with a spare lurking in my spares locker, and voila! While you are mucking around in there, it might be a good time to clean up the float valve, if you haven't already replaced it with an electronic one. Good luck!
Adam