it fails to fully seat a new round into the chamber.
Hi 44 mag,
Your issue may be that the spring in the Pmag is a little tight. Try loading it with 28 rounds and see how it does. Also, you may want to be a little cautiuos of loading mags on a closed bolt. Even my loosest mags will occassionally not seat all the way, which would really suck if SHTF.
I'm not very good at the online diagnosis thing, but that doesn't exactly sound like a mag issue (although you've said you don't have the same issue when using other mags). I would think that the bolt has more to do with fully seating a round into the chamber.
Your mag well could also be out of spec causing compression of the mag body as the rounds are shot.
44 Mag, how tight are PMAGs when inserted into the Magwell?