You might want to start using virgin brass, I had the same thing happen with new barrel from McGowan, it would not chamber pre fired brass from another rifle had to start using virgin brass.
This happens with my custom 8" twist 270 WSM. The smith used a really "minimum" reamer for it and though headspace is fine, my RCBS dies don't size the base of cases I fired in the original factory (win M70) 270 WSM barrel quite enough for resistance-free cambering. Said brass also gives a false "stiff bolt lift", which would feel like an overpressure load if you hadn't felt it on closing.
I use virgin Hornady brass and it works great for LOTS of reloadings. Very impressed with that brass. Wish I could find another couple boxes of them. On the wait list at a bunch of vendors.
Good luck with your 338-06. Great chambering.
Rex
