I have had a host of different issues with DPMS factory LR-308 mags over the years. Some are run 100% while others are FAR from it. I've had issues with mags having feedlips out of whack, as well as mags where the mag catch holes had been cut out of position in relation to where they should be...either of those could be causing your issues.
First and foremost...ensure that you actually have the mag seated completed in the rifle and that it is fully locked in place. Once that's done, have a look and see if it appears that the magazine truly isn't being allowed to insert far enough from the lower into the upper receiver to permit proper functioning. Also, with a loaded magazine, see if the rounds are resting against the feedlips of the magazine completely. If the rearward-most portion of the feedlips are bent or otherwise out of spec, they can cause the mag itself to push downward on the rear portion of the case body and cause the issues you are describing. Aside from those couple things...not a whole not more you can do at this point.
I also second the getting more (DIFFERENT) magazines recommendation. The Magpul 20LR series PMAGs are excellent. Magpul is intro'ing a limited capacity, 10-round version in the coming few months if you want to or you can wait that long. If not, and you can get someone to ship them to you, cutting down a 20-round Magpul mag and pinning the floorplate on to "manufacture" a CA-legal 10-round mag can be done fairly easily with simple tools (you'll need them shipped to you as a "magazine repair kit"). As sentry1 said...before the last craze of buying some places like MidwestPX offered cut-down and otherwise limited capacity 20LR PMAGs, but since the basic factory mags have become so scarce and expensive (double retail give or take), nobody has the limited capacity, modified mags available right now unless you stumble across some here or BARFCOM.
