I have a new one. I have old style American Standard valves on my tub
in the bathroom (separate valves for hot and cold). About 6 months ago
the cold water wouldn't run even after the stem was turned all the way
out (on). After replacing the top-hat washer it ran a little bit. Then
it started the same thing. I just repaced the entire stem assembly but
I have the same problem. When I tighten the retaining nut down enough
that the stem screw will turn properly (and not spin the whole barrel),
the washer is apparently seated even when the stem is cranked all the
way out (on), and no water will flow.

Any suggestion??? This is a weird one.