Bath/Shower Valve Not Turning Off Bath Water

My bath gets water when I turn the valve open to use the shower. I just went to ACE and the helpful hardware man sold me a new washer, but is still has the problem. Anyone have any ideas? Thank You!

If this is a typical 3 handle tub/shower, the middle diverter stem has TWO seals....a flat washer on the nose, and another gasket BEHIND the nose. The latter is the one which keeps water out of the tub when "shower" is selected.

If you could post any pics, we could elaborate.

You have the wrong washer...that blue washer should be flat and not a cone washer. But the real problem......the back-seat white nylon gasket is MISSING. This is a very common Price Pfister. Take it to the store to get a complete new stem assembly.