Alan,
I've had PMK part B precipitate a lot of material out of solution before, likely because of cool storage conditions. If I can't get things to go back into solution, I'll mix up a new part B from scratch (I've got Kodalk and Sodium Metaborate on hand - yes, they're the same thing, but the old Kodak Kodalk bottles are vintage and still work just fine).
And, yes, for some reason, once the part B precipitates a lot of material, for whatever reason, I sometimes cannot get it back into solution.
If you notice any kind of reduction in developer activity, I'd recommend mixing a new part B.
And, if there is any kind of solid precipitate in part B, either carefully decant or filter it when using to avoid particulates in your developer.
Best,
Doremus