I had a similar problem with a new .308 barrel I had cut and put on a Remington action. The brass I was reloading was full length resized and it would not chamber. I ran the brass through a set of small base dies and that did the trick.

I see you figured it out.
I once had problem like you are talking and it was a really small burr around the casing where the bullet goes in. Took a piece of emerycloth and knoked the burr off was fine. Found out some dummy didnt deburr after resizing.

has the brass been trimmed to length?