Just my tuppence.....

Could it be that the bolt carrier isn't moving back its full travel, in other words "short stroking" as sometimes described?

Because of the sometimes slightly shorter rearward travel the ejector is having less (or no) effect and at other times the short stroke means a lesser effect from the springs giving lower forward inertia to chamber a new round from the magazine.

Does the new round failure to chamber happen on a full or nearly full magazine, when the mag spring will be most compressed and thus more force needed stripping the top round?

It might be worth checking that the gas piston is clean of any fouling, that it isn't slightly bent and that the piston tube is properly installed.

If this all checks out, you could run a test with only two rounds in each of your magazines and see if they run reliably. If it does try more rounds in a magazine and if the problem returns, turn up the gas setting to see if that makes any improvement.