One of two things come to mind, extremely hard brass or doughnuts.
This is a dognut A little ridge of brass that forms inside at the neck-shoulder junction and if bad enough will prevent the expander button pulling through it.
If I suspect the little buggers are forming I use a pin gauge to detect them but you can feel them with straightened paper clip slid down the inside of the neck, you'll feel a bump as it passes the neck into the shoulder. Of course it they're pronounced enough you can also see them with a eyeball.
To remove them I'll push them to the outside with a mandrel then neck turn just barely nicking the shoulder to cut away the thicker brass. K&M makes a turning mandrel with a reamer to cut them out.
http://www.kmshooting.com/catalog/pi...388695985.html
By the same token, if you neck size with a bushing or Lee Collet die ( no expander) and the base of the bullets are seated out past the junction they're not a problem. If a bullet is seated in them when the round is chambered it can restrict the release of the bullet then bad things can happen. This is one result of an undetected doughnut.
If its simply work hardened brass annealing will fix it.
Bill
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