If it’s happening with two guns that fire other, cast bullet loads and factory loads OK, you’ve pretty much removed the possibility that it’s the gun. (Other than the extractor dictating headspace) There’s something about your recently loaded ammo that’s causing the problem. I would guess that something is preventing the firing pin from reaching the primers.
…Cases too short or a slight crimp allowing the case to seat into the chamber a little deeper if the extractor hook is a tiny bit farther from the breech face. Is the extractor hook within spec?
…Are you you using the same case brand? If they’re a different brand, is the rim thickness the same as the cases of those loads that work OK? Thinner rims may be allowing the extractor hook to hold the cases a tiny bit farther away from the breech face or the firing pin.
…Are the primers seated deeper than those in those cast bullet loads? Just hold a thin straight edge across the bases and eyeball the depth of the primers or measure if you have a tool to do it.
Hope you get it figured out.
