Blue, turn your resizing die in another 1/2 turn and see if that doesn't make your problem go away. Either your shoulders are being pushed forward or the shoulders are being pooched-out (highly scientific term meaning made fatter) on your neck sizing stroke.

Blue, what some of the other posters said. when you size but don't hit the sholder the headspce dimension often lengthens. As you described it that sounds like what the issue is. Expander tension can also definitely pull the shoulder forward as well, as some others mentioned. If a piece of your fired brass will chamber--which it certainly should--then it is either one of the two previously mentioned in all likelyhood. It is easy to check on the sizing, just screw your die in a bit, I'd say at least a 1/2 turn past touching your shellholder, but I have had to go as far as a couple of turns with pretty hard caming over of the press. If you want to test the expander tension (pulling shoulder forward) issue, just size a couple without your expander in the die.

Regards--Don