Don't know if this will work for you, but it is a simple fix if it does. I had an occasional failure to fire with my sidelock rifle and my shotguns. But when I did they would almost always go off if I re-cocked the hammer and tried again or maybe a third time. On advice from a knowledgeable gent, now when I cap the gun I GENTLY set the hammer down on the cap and then push pretty hard on the back of the spur to force the cap all the way down on the nipple. I have not had a failure to fire since I started this practice. Of course be sure the muzzle is safe, but I have been doing this for 15 years now and have never set off a charge while seating the cap this way.