Black powder sub powders. The bulk of the granules that clogs up the powder channel and doesn't allow powder to get in under the nipple, sub powders in general that require 700-800* spark in order to reliably ignite.
I was and still am a big fan of Pyrodex. Its a great powder that's consistent and reliable but even I know now after shooting Goex for well over a year now, it is much more prone to hang fires or fail to fires simply due to the reasons above.
Sub powders also degrade. How many times have we seen, I have a pound of pyrodex or triple 7 thats 8 years old... Is it still good?
Introduce swabbing between shots and it all adds to the FTF.