I have yet to lose a case in the 6x55. I have had similar failures in the 6.5 WSM, always on brass that hasn't been annealed after case forming.

Take some measurements:

1. Case neck OD loaded round.
2. Case neck OD after firing.
3. Case neck OD after sizing with the expander plug removed.
4. Case neck OD after sizing with the expander plug installed.
5. OD of a can of Coors light. You will need to empty this can during 1-4, it must be empty to ensure there's no stretch due to contents.

Lets see how much you're overworking your brass.