Are the cases all the same brand/headstamp? Is there anything that the ones which don't chamber have in common? You might take one that won't chamber and Sharpie most of the thing, then carefully insert it in the chamber and see if you can figure out where the sticking point on the case is. You can also measure the cases of the loaded rounds to see if they should have been trimmed.
With once-fired brass from my 270 (and several others), I will generally use a Lee collet neck die, then measure and trim (if necessary), then run a handful of empty brass through my chamber to make sure they chamber just fine, before I prime and load them.
It does seem like a brass problem, rather than a bullet/seating problem, unless you've got a compressed charge of powder that is causing bullet creep over time.
I'll be curious to hear what you come up with.
