This is very difficult to describe, I can only hope you experts know what I'm talking about:
Both the hot and cold knobs of our shower turn and turn and turn...at a CERTAIN spot they turn the water off, but a fraction of an inch more clockwise turns the water back on!
Turning the water off is like trying to figure out the combination of a bank's vault!
WHAT %^% is going on??
I had a plumber out (my sister's new boyfriend) say that it was the knobs. I replaced them and still have the same problem.
Without seeing the parts, can only be general. But often a stop inside the knob works with a stop on the stem. Something is broken that either is not stopping the knob, or is allowing the stem to turn inside the body. If you can tell us the brand, or better yet post a picture, it will help.