If you have a behind-the-seat tank, you have the pipe coming out of the top of the tank at the sending unit. Your fuel pump therefor needs to pull fuel up hill to get fuel out of the tank. I small leak in the fuel line between the fuel tank and the fuel pump will suck air, thus preventing the fuel pump from pulling fuel.
Check all fittings for tightness between the top of the tank and the fuel pump.
An empty fuel tank will cause your symptom as well.
