An idea maybe, to verify your water tube IS in fact sliding into the grommet on top of the pump, and further, exactly how far it's being inserted.
If you have an O-ring assortment and can find one that will fit that tube, grease up the end of the tube so the O-ring can slide easily on it, set it about 1/4" from the end, reinstall the lower, then pull it to reveal how far the o-ring is sliding into the grommet. Should only take a minute, and can verify there's nothing going on there beyond doubt!
If you can't find an 0-ring, maybe a rubber band would work...