Case mouth not being closed enough by die

Heres a long shot - is the expander ball sitting too high in the die? On Lee dies this can be a problem and it typically happens after a jam. The expander is held by a collet which slips when distressed.

The neck should still be sized down by the body alone. Sounds like the expander ball is currently out of the picture.
 
If it works properly for some cases and not for others, it is a problem with cases, not the die. I suspect you have a mixed lot of brass. Use the cases that work and recycle the rest.
 
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