If you are hand cycling you need to make sure you're releasing the charging handle when all the way at the back and not following it down.
Likewise, all my Grendel's would feed at least 5 shots out of a plain PMAG.
If you can't find known good Grendel mags to test with I'd try a pmag from a buddy.
Mine will actually shoot about 10 Grendel out of a 30 round 556 pmag, but the stacking gets wonky.
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