[Unclenick]

That might be a lube migration issue. Try soaking the lube off in mineral spirits and dip in Lee Liquid Alox and let dry.

The primer could also be the culprit. I've not had any of the S&B primers, but have heard a couple of people complain they were hard to seat, like the Russian Tula and Wolf primers. I do have experience with the Tula primers, and the issue is that the edges of the cups have burrs, so they have to be pushed in pretty firmly to go all the way to the bottom of the primer pocket. You may have had a primer that didn't seat fully and so the firing pin strike knocked the anvil loose, the priming mix broke up, and you got ignition flame that was weak and spread out over a longer total burn time than is usual. It's a known phenomenon.

Another factor is that 231 is an old St. Marks ball powder formulation. Even though it's a fast powder, it likely needs hotter sparks than, say, Bullseye or Red Dot or Clays or N310. If so, you want a primer that has some aluminum powder and Barium Nitrate in it to get hot sparks. Most any domestic brand will do. Additionally, you have a case that has some empty room in the powder chamber. Sometimes you will get better ignition consistency and performance using a magnum primer in such a case. The main purpose of a magnum primer is to make more gas than a standard primer, thus to better set the start pressure up in a larger magnum case. But a lot of empty space can require more gas to pressurize and ignite well, too.