It looks like there was a crimp issue. I lowered the crimping die very slightly, and the rounds started going in.
I guess the die must have managed to back out after the last group of cartridges. I made 450 cartridges, and only the last 50, which came from the same box, had this issue.
I didn't think there was any way for the crimping die to move, but it looks like I was wrong about that.
While I was at it, I checked the sizing die, and there was a tiny gap between it and the shell plate, so I fixed that for next time.