I recently picked up a Stag M8 (like just a few weeks ago). It came with a metal GI mag, and I wanted to ...alright, self-admitted tacticool junkie. I wanted PMags. I bought four of them new in the wrapper from my LGS. Couldn't tell you the manufacturing date. I tested each of them both empty and loaded and they all dropped free up to about 45 degrees off vertical. No problems. I finally made it to the range (outdoor, in FL, ~95 degrees) and ran a full 30 rounds through each mag (still dropped free). The next evening after a thorough cleaning and re-lubing, I started dry-firing practice and none of the PMags dropped free! The GI mag still does, but the PMags don't. I loaded one up with 30 rounds, and it still wouldn't drop free. I had to forcibly yank each one out. I know (googled) that early PMags had problems in some lowers and a little bit of sanding on the mags cures the problem. In this case, the mags did drop free. Did the magwell shrink, or did the pmags permanently expand? Is it worth my time to sand down the mags, or should I try to exchange them?

Check to make sure there isn't any gunk in the mag-well thats holding up the mag release. See if there are any drag marks on the mag, could help indicate where and why they are not dropping free.