The higher charge load may be making the brass not shrink away from the chamber mouth adequately before the bolt unlocks and extraction begins. That in effect delays the cycling a little bit and possibly enough to lose the momentum needed for the bolt to travel all the way back. IE not the gas system's fault. Its trying its heart out to cycle but the case is still holding on a little. Usually however, you see pressure signs in the form of case head growth when precisely measured. See if those problematic rounds cases will slide back into the chamber using your finger to push them back in. If they don't or get real snug, you might have subjected them to excessive pressure. Clean the bore and the wipe down any film on the outside of the cases with some mineral spirits before trying to reinsert them, to make sure all you're feeling is resistance (if any) due to case head growth. I sort my brass primarily by the fattest diameter I can measure with a micrometer. That usually occurs just above the web. My rifle will not chamber rounds in excess of .5017, .5000 being SAAMI spec for the caliber at that point.
Hoot
Edit: In case you're wondering, resizing the brass will not remove case head growth.