I've tried all the tricks I've read over the years on my mountain rifle and never did get completely reliable ignition with Pyrodex. Pretty good but not perfect except for nipple charging with black powder which is a PITA and has its own problems. Possibly I could have worked it out but I decided to stop beating myself over the head and went back to black powder which fires every time. Saved the Pyrodex for weapons where it works more reliably. I don't think it has to do with leaving it loaded. Rather the breech geometry and that Pyrodex has to get hotter to ignite.
