Well the knob wasn't working very well - I couldn't get the bathtub to work at all - so I played with it a bit. I turned it one way and I turned it the other way and I turned the water on and off. And that process seems to have made it worse. Now the water is stuck coming out of the hand-held shower and turning the knob does nothing.
Actually, as I turn the knob it feels like the knob isn't fully engaging. That same feeling you'd get if it were stripped just enough that you feel the pressure as you turn but then you feel a slight bump as the knob turns around over the nipple it's attached to. Well both the knob and the stem (is the image below called a stem) and nipple (what the stem actually goes into way inside the wall) looked good to my naked eye but to test I took a knob from another shower and tried it here and had the same effect.
So my question is what do I do now? Is it possible this issue is deep inside the wall? I mean I want to redo this bathroom eventually but I'd rather not right now. Is there any way to fix this?