For future reference, the other two Dillon 550B primer issues that show up with higher round counts are:

1) Primers not fully seating.

When this happens it usually means one of the two small hex head screws that hold the shell feed and primer chute assembly to the bottom of the plate on the ram have come loose and backed out. The plate then stops it's downward motion too soon to allow the primer to fully seat.

That one drove me nuts until I figured out what was going on. Blue Locktite is your friend with that one.

2) Primers not being pushed all the way out of the case.

With Dillon presses and dedicated tool heads, you can often load tens of thousands of rounds without ever removing dies from the tool head, which means you miss little things that are otherwise obvious - like the end of the de-capping pin slowly unscrewing until it just starts to bottom out on the base of the case.

Since it feels funny operating the press, you'll catch it and you'll adjust the pin assembly upwards, and duly note that the de-capping pin now has the proper projection below the die.

It'll work great to de-cap about 95% of the cases, but the other 5% won't drop free. The pin protrusion still looks good and you'll miss the problem unless you press up on it with your finger and note that it rises an 1/8" or so.

You'll only have to do that once before the light bulb comes on.