Two possible things:
1) How old was your mixed pyro? You have maybe a 15-20
minute window at most after mixing it before you use it.
2) I had the exact same problem with some Xtol once, and
figured out that i had accidently dumped an ounce of fixer
into a quart of Xtol stock. I had poured out too fix, and
was pouring the reast back into what I thought was the
fix container. Point is, if you have any cross contamination
at all, it does not take much to ruin your developer.
If you had under or over exposed the film,
you would have some kind of image. But a completely
blank film is either a camera problem - not here, since
you used both 35mm and 120 - therefore, two different
cameras; or it is a developer problem. Maybe some
more specifics on how you are mixing your PMK,
developing, etc.
One last point - I have ruined far more
film over the years than I care to admit. Don't give up,
you'll get there yet.
joe
http://www.multiboard.com/~joneil
B&W, Large Format Images From Southern Ontario