That is nothing I have ever seen, have you checked the diameter of the bullets to see if all are the same? Why would brass not expand to the size of the neck in the chamber when fired.. Have you verified the thickness of each neck that will not pass a bullet versus ones that will? I am curious to see and hear what is causing that problem, are all the same brand and lot on the cases? I would sort out all the ones that are fired and allow the bullet to flow in and out of the neck, then I would weigh each case and match those cases that are the same, reload those and see if the group closes.