With that type of breech, I put a countersink in my drill press and slope the mouth, then file the edges round and polish with sandpaper. How much to "cone" depends on the bore size: I go up to a little short of the rifling, so there is a sealing seat. The wall of the breech should be drilled through at tap drill size, maybe just a little smaller for the liner to seat, but definitely as big as possible. I try to smoothe the inside shoulder of the passage hole as well as I can. Have considered cross-drilling to enlarge/cone inside of flash hole, but never found it necessary. All this is to get powder to fill chamber and flash passage, and I'm happy with results. They work better than flat breech for me.
I doubt the flash hole position is the problem, more likely powder not getting close to hole.