David Emerson said it: maybe die too long.
My story, short form: Radical Firearms (Latin for ***), 6.5 Grendel. Tough to chamber Hornady factory ammo. Gsmith touched up chamber, fixed. Still tough to chamber reloads. Die set properly, slight cam over on press. Not quite sized properly per case gauge (didn't have that when I started reloading this cartridge). Spun up on a lathe, trimmed maybe 5-10 thousandths off die. Works fine now.
So, the relevance in that story to your situation: could have been the chamber. Not being a Radical ***, not too likely, and chambers factory ammo fine. Resized cases not fitting, maybe die not set right (operator error, doesn't seem likely) or die a tad too long (more likely I would think). Minor trimming of the die should harm nothing. If it's a bushing die, you could remove the bushing, set the die to bottom out on empty, then try "sizing" a factory loaded cartridge to see if there's any resistance. That might indicate that the die isn't low enough to even contact the factory case. Not sure if that's really necessary. Just size a case and try to chamber it empty.
FWIW.
--HC