Use a vent hole size of 1/16" for 3f and a 5/64" for 2f powders. Pick the vent before you load your next shot and make sure your pan is dry before you put your pan powder in and wipe the underneath of your frizzen as well. Black powder residue will draw moisture within a minuite or so. It may look dry but the fouling will draw moisture. Like the others said bank your prime away from the vent hole.

There are lots of tricks to the flinters and I could go on for a while. Stick with it as there is no greater reward than hunting with flinters, with the possible exception of stickbows.