Having dealt with this issue I narrowed my problem down to the ammo, one particular brand, B & P Comp One. I first suspected the gun, then found it would happen in every gun I owned with the exception of two external hammer guns, one double and one single barrel. It boiled down to gunman's no. 1 point, about the rim depth being too deep, but in my case only too deep for the particular shell brand in question. However, and again, not the gun's fault due to chamber rim depth being cut too deep, but the shape of the rim on the shell head itself.
I had bought two or three flats of those shells to use in my vintage doubles due to their light recoil and low chamber pressures, and had reached the point that I figured to just use them up in the external hammer guns, until a member here private messaged me a solution that would allow me to be able to use them in my other vintage 12 ga. doubles ............. I went to the dollar store and bought a package of tiny rubber bands that women use for their hair, as I recall. They are about 1/2", or less, in diameter. They stretch over the head of the case and you slide it up against the rim. When seated in the chamber the tiny rubber band holds the case head snug against the standing breech and ignition is 100%. You get a blue million of the little thingies for about a dollar.
Have you attempted to isolate the issue to either the ammo your are using, or to the gun itself ?
