Hi, Natalie. I'll take a guess that part of the rubber washer was missing when you replaced the old one on the diverter valve. Either that or you did not have the stem turned in the right direction into the bonnet nut when you put it back together. Ask your father to check it out again. I would not be surprised that there was a chunk of rubber from the old washer blocking something. When you put it back together, make sure that the stem is screwed as far into the packing nut/bonnet assembly as possible.