Peter, I've got four ideas. First, do you know the real world film speed of PL100? I don't, and sometimes it could be significantly less than the advertised speed. Second, is it possible that you set something incorrectly? I know that I've done it, especially if I had to really hurry to get the exposure. Third, it's possible that there some technical problem with the the shutter or aperture. Fourth, you could have experienced Xtol failure. If so, the image should be very faint.
You might try taking an exposure in a controlled setting, and developing again. Maybe do two sheets, one at 100 and one at EI 25. You could also test the Xtol if you've got a film/exposure/development combo that gives good results. (This would be to check if the Xtol is bad.)
I hope that you figure out what's up.