On each press of the end of the stalk can you hear the pump running? May need to have an extra pair of hands or ears for that! I don't know if it's worked fine on the days in between of course! It's just a guess but it could be the little non return valve in the line, leaking back a small amount, so that you almost fill the line the first time, but not quite, depends how long you ran the pump for. And sadly I don't know where that is, not had a shufty on ours yet!