Cause of a rotor sticking only while playing?

[Donn]

I believe Mark and Joe have gotten this one right.

Did I miss something - Mark and Joe explain why a valve might stick only when playing?

[Michael Bush]

There is a lot of lore here, and I don't want to call a halt to the discussion. In this particular instance, it looks like the dirt-or-lint theory seems the most likely, because I just finished the third performance of the day, and the valve didn't offer to stick once. I must have wiped something away yesterday without noticing it.