If your seating that bullet wrong or your dies are closing the neck too much, your bullet could be cantered a tad.

Run your case thru the die without expander. Measure the inside and outside of the case with calibers. Next run it thru the expander. Do the same thing again. Next, load the round. What you should see is something like this.

For example. 308win

Fired neck inside:310
Sized neck inside:.303 maybe .304
Ran thru expander:.305 or .304

You can check your expander ball with calibers too to make sure they are in spec.