I recently purchased a surefire 12 round mag and had the same thing happen.
I sent it back to surefire and within 7 days I had a new one at my front door. The first thing I noticed was that they changed the profile of the front lug, it now looks more like the 5 round factory mag. Its actually a little thinner and is tappered toward the front. I have put over 200 rnds through it without a problem.