Ah, that is about when I noticed a few years back something about wiping between shots as I do, and still do, too.
I had a time that I began getting flash in the pan; checked the flint, cleaned the frizen (I carry a small bottle of alchohol soaked wipe patches - just t-shirt). After a lot of fussing I got it firing again, then pan flashes again...it was driving me nuts!
Not sure how but I found that after a few wipes if I turned the barrel down and smacked the rifle with my palm several small chunks of smutt fell out - I could see them as I was using a horse blanket to load over off the back of my car.
SO, ever since then I will turn my rifle muzzle down and smack her a couple times after four or five shots, more often as I shoot more - and every time I see that little bit come falling out.
My conclusion; that smutt was building up and eventually blocking the vent...also, I pick every time but occasionally on long days I will when I see things getting dirty...and about 10 or 12 shots I take the time to wipe the frizen, flint, and pan with an alchohol wipe (rest of the time just my finger and shirt), and for accuracy I will run an alchohol wipe down the barrel followed by a dry wipe (*use an Under Sized jag!!*).
I have not experienced pan flash in a few years now other then a worn flint!
Just some ideas I have ran into and what solved them for me - everyone's miles may vary