I had a similar problem with my sidelocks when using Pyrodex RS. I would have random hang fires and even some misfires. when I switched to Pyrodex P all that went away. It went from around 20 to 30% to less than 1% of the time. As mentioned, I think it has something to do with the powder getting down through the fire channel to under the nipple. 2F seems to have more trouble doing that.

Hope this helps.