Will it close on just the resized empty case before you seat a bullet?
You might have a tight chamber. I had to lap a 450b chamber once that was out of spec. Talk about a labor of love!
Can you mic (not caliper) the widest part of some of the cases that won't chamber? In a perfect world, after one firing and assuming they were from 260 gr factory fodder, they should come in around .5000 to .5005. Possibly less. I'm assuming they were fired from the same gun when they were new? I'm no fan of Remington 450b ammo but maybe they've gotten better than back when I tried some of the Accutips when they first came out. Also, I'm assuming you checked that the resized cases did not exceed 1.70 inches in length. BTW, Lee or Hornady dies? I ask because a few members had issues with their Lee 450b die set and had to get them replaced. I don't remember why other than there was a sizing issue of some sort.
Hoot