My water pressure was low just in my shower so I replaced the head. Still have the same problem. Local hard ware store said it could be the lever to switch from bath to shower. I'm having a problem removing the plastic piece that moves left to right for bath to shower.

STOP!! Before you tear into your shower valve unscrew the shower head. See a rubber washer with holes around the edge. This is a water saver and cuts down on trhe flow Pry it out and toss it. Does that make the flow better? Let me know Tom

Healy... is that a symmons tub/shower valve, by chance..?

Did you try Speedball's idea? Let me know.