I assembled this 'cheat sheet' few years back on substitutes.
As for your problem it is likely not the flint being dull, more likely it is the substitute powder you are using. You can use it but it often requires a booster charge in the barrel to ignite it. The pan sets off the booster charge which ignites the substitute. If you are dealing with percussion caps they are hot enough to ignite the substitute powder.
Quick way to test your flint. Put some powder in the pan and trip the hammer. Does it flash, if yes then the flint is adequate, if not then there is a flint problem
Loading up the pan is unique to each, not only do you have the width of the lock but the alignment with the touch hole in the barrel, also the breech plug makes a HUGE difference. When working with new rifles, loads, etc.. it is very good to take some type of hat pin or other small object so you can insert into the touch hole and crunch the powder in the barrel, you can quickly tell if there is powder at the bottom of the breech plug.
This is a patent breech on my kentucky rifle.
Now that you see how DEEP it is, lets look inside the barrel and how it is shaped.
So, powder needed must equal volume of the breech cup or exceed. If it is less then you run into dangerous conditions.
There are many styles, shapes and types of breech plugs out there and to really optimize loading you need to figure out what type it uses and how best to incorporate that into your loading, firing and cleaning scheme.
Oh and before I end I would like to say congrats on owning one of the first model 'black rifles'
Ed
