IT IS BUGGING ME ABOUT YOUR RIFLE.
please allow me to make a few comments and ask some questions , just to possibly help you put your finger on the problem.
what was the weather like on the 2 most recent failures compared to when you were at the range last?
are you using black powder , pyrodex powder or pyrodex pills? if it is the pills, are you sure you are putting the proper end toward the breech?
not knowing what your rifle is, by make and model, is it one with a firing pin and is it protruding far enough and that smoothly.this could go back to the weather. was it cold enough to slow the movement of the pin?
you don't have to respond to me if you don't want to i just wanted to throw these few things at you to help you find the problem.
one more,,,if it is the pills that you are using,are you seating the the bullet FULLY DOWN against the pills. a 209 primer is very powerful by itself. if the bullet isn't tight against the pill , it is possible for the primer spark and gasses to actually move the pills. right you don't want to crush the pills ,,,just a tight seating of the bullet. take care