Wow, lots of suggestions to reply to! I seriously doubt that the cases or chamber had any friction reducing contaminates on them. I've been paranoid about that after seeing smeared case heads and disintegrated primers result from someone moly coating their chamber (not me). Nevertheless, that is the most likely variable that I would have overlooked. There was a thorough cleaning before and after each group that was followed by an oil patch and a dry patch. I generally swab the chamber with alcohol before hitting the range, but I may not have fully stripped the chamber of oil this time. I'll give extra attention to that step next time out.
Cases are new, and I am bumping the shoulder with the die because factory inconsistency within the lot is enough to prevent a case from chambering without excessive force every now and then. I sized the second set of cases on a different day, so I suspect that it is possible that I might have had the die set too deep with the first round, although I don't think so. I cannot measure any difference in the headspace on a newly resized case and a fireformed case, and I have not adjusted the depth of the die. I did remove it from the press and reinstall it.
I'll post again after my next outing. If I have a positive experience, I'll assume that there was some cleaning error or other fluke variable that I overlooked. Thanks for the advise.