Can't figure out, and Duramag can't either, why their 28 round mag will lock back in my AR but their 20 round mags won't.

I sent the 20 round mag back to them and received two new mags, and neither of them will lock back either.  They feed perfectly though.  They say the mag I sent them locks back every time.

The new mags might be putting more upward pressure on the bottom of the BCG.  The added resistance/drag might be zapping just enough recoil energy where the bolt carrier isn’t going back far enough to clear the bolt catch.   Try one round in the chamber with empty mag, does it still fail to lock back?   If it still fails to lock back on an empty mag you might try a lighter buffer.   Your rifle might be a little under gassed or over gassed.  If your rifle is way over gassed the BCG might be cycling faster than the new mag follower can raise to catch the bolt.   Under gassed might not have enough bolt speed to over come the pressure of the tight fitting new mag with new strong spring.   If the rifle is under gassed a lighter buffer might help the BCG speed to where the BCG fully recoils and is caught by the bolt catch on an empty mag.

I run a half dozen of the CPD gen III (before they changed name to Duramag) 20s, never had a problem with bolt not locking back.
That’s in three different builds.
If you insert an empty mag then charge it by hand, does it fail to lock back?

What weight buffer? Std. buffer spring?