First off it's not the best idea to use fully loaded mags. I've heard several times that you only load to 90% of the mag capacity. IE 18 in a 20 round & 27 in a 30.
Were you chambering by releasing the bolt with the release on the side or with the charging handle? The better way is to lock back the bolt, insert the mag and then slap the bolt release.
My Dept. had a few problems with brand new guns doing a similar thing and the vendor ended up replacing a few barrels and swapping a lot of magazines. We went to thermolds and they seem to be working fine for us and now that the guns have a bunch more rounds through them, the original mags are working a bit better.
Hope this helps you out.