Another problem could be a mistake that I have seen a lot of guys make.
After hunting all day out in the cold, they bring the gun home and into a warm house, first they put the gun into their vehicle with the heat turned on high without the gun in a gun case, this will cause the metal to start sweating and foul the fuel if you will, and the next outing you will hear them cussing because the powder wouldn't fire.
If you plan on leaving a charge of powder in your gun and take it back into the field say the next day or something, its best to put the gun into a gun case and if nothing else, put it on the back floor or in the case of a car, in the trunk or rear hatch and wrap it in a blanket or something, farthest from the cars heater, and leave it in the car where it can stay cold to keep the metal from sweating!
One other thing, before loading your first load, snap a couple caps through the breech in case there is any left over oil that could be soaked up by the powder
I am using an old Huntsman .45cal from 1972 with a standard #11 cap, I use 777 FFg powder with CCI #11 Magnum caps and have yet to have a problem with ignition!
Ken.
P.S
Please disregard all of this if you already take these precautions, I sometimes tend to ramble on about things that most hunters with BP guns already know.
I think I tend to revert back to when I was teaching my kids about hunting and get carried away!