Before hunting large game, or shooting a match I've tried the following;
1. Slap the buttstock after dropping powder, it settles it in the breach/drum.
2. Pull the nipple, trickle powder inside the drum/bolster. I fill it full, crushing the powder as I install the nipple.
3. At matches is very common to snap caps, then pour a charge of powder down, then keeping it pointed up and safe, cap again and fire. I've never seen one fail to fire after this.
4. Be sure and wipe the oil from the bore before snapping caps and loading. The oil makes it very hard to fire... trust me, I know.
Ps, I've SEEN this work. Use magnum caps. I loaded a new to me 20 bore, after snapping caps and slapping the stock it failed to fire. I asked the guy next to me if he had a worm to pull the load, he handed me a tin of magnum caps. Both barrels went off first attempt with magnum caps. I ordered 2500 magnum caps that evening.