I have a stock factory model7 in 243. I have owned the gun since it was new. Right out of the box it needed a new barrel, the first one had a bad chamber and would cause case head separations. I rarely shoot this gun, but have recently decided to reload some 100gr's for whitetails. I have never had a problem with factory ammo chambering, but today I can't get it to chamber a re-sized round to save my life. Tried a Forster FL die and a Redding body die, still no go. Fired a factory round to get a fresh brass, it's fits after firing, but won't fit after resizing with either die. Damn, I think the chamber sucks on this thing still! On some brass it leaves a small shinny ring just ahead of the case web.
I've only been reloading for 6 months ... so keep that in mind. I've had a similar problem. I wasn't setting up the resizing die correctly. I backed it out 1/4 turn after touching the shell holder in the full up position instead of turning 1/4 turn further in....
Take a fired case but unsized and roll it across the table and see if it wobbles worse than the tires on your grandad's Powerwagon. Then stand two case up on their bases so they are touching each other and slowly rotate each and see it the cases are still perpendicular to the base or if they look like the leaning tower of Pizza. If so you have an out of alignment chamber.
Finally, and most likely, see if you are getting the shoulder set back by the die. Some time the RCBS dies will bottom out on the shell holder before they move the shoulder back.
I ran into this problem with my .280 dies trying to get the proper headspace for my prohunter. I solved the issue by taking a diamond stone and honing .003 off the shell holder. It took about 10 minutes and really did the trick. I would highly suggest that you take your time and hold it square. I was surprised how uneven the original shell holder was prior to this process. It is nice and square and works as intended now.