I also recommend trying graded paper, and if you're doing the afterbath, leave it out.
You also may not be agitating enough at 10 sec. per minute. For tray development with PMK or ABC pyro, I agitate constantly, meaning I constantly shuffle through the stack, unless it's only two or three sheets, in which case I might shuffle through the stack once every 15 sec.
Another possibility, if you don't do much tray processing, is fog from light leaks in the darkroom. It shouldn't be a big issue with FP4+ in PMK, but nonetheless, it helps to take extra precautions against light leaks if you're going to have the film in an open developer tray for 11 minutes.