Maybe an issue, already mentioned above, is the pan on this lock. The purpose of the pan is to hold the priming powder that is ignited by the sparks generated by flint striking the hardened frizzen. The larger the surface area of the powder, the greater is the probability it will catch a spark and be ignited. An observation of original pans has shown some interesting shapes:
(From Dixie Catalog, 1970s)
It appears a good relationship is a wide, shallow pan with a touch hole at the top of the pan. Kibler realized this when he made his lock for his Colonial Rifle:
So the first thing I would do is open up that pan to the largest area possible to provide the greatest surface area for the priming (and I am a 4fg fan, too.)