HI Aqua
If it works underwater, then the pump-problem is solved. Your problem is the "PRIMER" . If there is NOT enough water in the chamber, it cannot pump. When I bought mine, the primer, looks like a suctioncup, was bent out of shape. But it workes. Needs a few more strokes than with a good, straight one. There is one more thing you can try. Don't laugh now. Get the outlethose ans suck until the water flows. That is ONE way of priming. Let me know how it works. BTW, if there is air in the pump-chamber, it will NOT work, the pump that is.
John