You aren't using pellets are you? That would do it. Ditch the rust-causing pyrodex, clean REALLY well and then move to loose 777. Definitely bump the stock on top of your shoe and twist it to shake a few grains into the channel. Rarely have a problem with these hang firing. You can get "hotter" caps too. That should do it. When you clean remove the nipple and use a small pipe cleaner in the flash hole. Don't let oil and lube pool in there. Clean the nipple well too, use a small wire pick to ensure hole is clear. You can experiment with different type of nipple too, some just work better than others.