This pump works by pumping air in, not fuel out. Since air or gaseous fuel is much more compressible than liquid fuel, it will take longer to pressurize the emptier it gets. Seems like filling the fuel back up would tell you whether that was the problem, otherwise it's probably broken. Make sure it's screwed on right too. You will also want to be sure that the internal fuel cup in the pump is lubricated - pull it apart and put a bit of oil or spit on it to make sure it's making a good seal.