Pyrodex P is an excellent powder, All pyrodex is in fact. You're probably having a nipple problem. Its not as bad as it sounds

Some times its hard to get the powder to shift over into the snail/drum and under the nipple. Other times, Its the nipples fault.

I like the Hollow base spitfire, Knight "Redhot" I think its called. Those are my two favorite to use. 4 of our sidelocks use the hollow base spitfires all because the factory nipples caused nothing but hang fires.

CCI #11 magnum's also help if you can find them.