Blanking primers often sends a little disc of primer cup back into the bolt bore where it can hang up, get ironed to the side, get into the spring coils etc raising all sorts of Cain.
I'm sure you've cleaned and dusted your bolt Pete but my question refers to those pesky primer cup slugs that can lodge in the smaller portion or shoulder portion of the bolt bore and get swaged/ironed/hammered into place such that they impede firing pin travel but are hard to see. And don't come out without a brush or even a picking tool.