All good advice but here is something that no one mentioned:
Several years ago I had a similar problem with a 6mm Rem rifle and standard FL sizing dies. I checked everything repeatedly and could not figure out why the resized brass chambered tightly. In the end, I determined that the neck sizer button was actually pulling the neck/shoulder up as the button was being pulled up through the case during the sizing operation. Cleaning and lubing the necks helped but did not eliminate the problem. A replacement button, along with clean and lightly lubed necks fixed the problem once and for all.
You may want to try sizing a few of the problem cases with the expander button out of the die and see if they fit in the chamber any better. It's an easy check.
Personally, I believe that neck expander buttons are always a potential problem as far as inducing concentricity problems in reloaded ammo so I do not use them any more. All of my dies are bushing dies of one brand or another, mostly Redding. Using the proper bushing size and cleaning necks works great without the need to lube the inside of the necks. It's the expander ball that makes inside neck lubing important, IMO anyway.
