jimbobborg
Oddball caliber fan
I clean the dies whenever I start noticing issues. The issue will vary, but it's always good to eliminate dies before looking elsewhere when you run a progressive. I don't know if you load cast boolits, but boolit lube would be a nice source of that buildup. Wax on FMJs to keep them nice and shiny? Whatever is on the bullet will go into your die. Do that enough times, and it will stay there and build up.
I haven't loaded lead in over a year, so this is kind of weird. I'm using polymer coated bullets now, I don't know what the deal is. It's not red, which is the color of the coating. I also didn't realize how many rounds I've loaded since I cleaned them last.