I reload 9 mm exclusively (so far) on my XL650 using Dillon dies except for a Redding micrometer bullet seater.  

The last time I reloaded, I had several rounds where the spent primer didn't eject all the way from the sizing die and was pushed back into the case by the new primer at the primer seating station.   This happened about 6 times in about 400 reloads and I discovered it by seeing the unused new primers in the little tray on the front of the loader.  There was no particular change in the "feel" at the primer seating station when this occured.

I'm loading well used mixed cases but no particular brand stands out as the problem and none are military cases with crimped primer pockets  The new primers are Remington 1-1/2 small pistol and they feed perfectly from the primer tube.   I always hear the "click" from the spring-loaded primer punch in the sizing die so I have no idea how the spent primers manage to avoid ejection.

Ideas and suggestions?