I realize it's a bit dirty, but the issue is this: There is a little round plunger-knob thing at the base of where the water fills the tub. When I pull it down, turning on the water at the same time, it activates the shower. Only about 5% of the time, when I pull it down do I get good water pressure. And about 85% of the time when the pressure is good, does something seems to click in the guts of the shower assembly, & the pressure goes down immediately. Do u have any idea what that is?