I am loading .45 ACP with Dillon dies in an XL650.  I am using random brass from a large amount collected over many years, so this shouldn't be an issue with case length from a different lot.

After setting up the machine for proper belling, I've loaded approximately 250 rounds with no problems, and then, BAM!, all of a sudden it went from a great case bell to shaving lead on the 185-grain LSWC, though bullet shouldn't matter.  I did load the first 100 with a 200-grain plated SWC, but there was no change to the belling in between.

There was no pause, nor anything different between those two strokes of the handle.  The powder/belling die is still tight against its lockring.  I can't for the life of me figure out what changed.

I even loaded over a dozen rounds, shaving the lead, just to see if it would go away, but it seems to have permanently changed.  I guess I am going to have to readjust it?

What could have caused this, and is this something that others experience?