If the granulation gets smaller, the powder charge length to diameter ratio gets smaller. When the column gets too small, ignition gets erratic.
It becomes a juggling act, lots of experimentation needed to get it to work.
I found a couple cardboard wads over short, fat charges under the projectile helps a bit with consistency, but that is probably a gun to gun variable that needs to be worked into the load.
Also, cardboard smolders and lights stuff (like my wife's garden) on fire. Soaking in a baking soda/water solution for a few minutes and drying in the sun before use seems to prevent that from happening.
Rick