After doing a LOT of research it seems like the 20rd PMAGS are hit and miss with problems, but I can tell you it's VERY common for the bolt not to lock back on the last round. The consensus seems to be the follower design, which is different than that of the 30rd PMAG.
I'm not going to even bother with sending my 20rd PMAGs back to Magpul because it's not fixable. It's the design of the follower. I'll just use 20rd USGI mags and problem solved. My 30rd PMAGs work flawlessly.
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