Sounds like you have built up crud in the powder chamber, and in the flash channel by not cleaning the gun properly, and not using the right powder. If the caps go off, and the charge doesn't ignite, its a pretty good indication that you have a blockage.
You are probably using too much fluid on that patch you run down the barrel between shots. And the jag you use may be too close to bore diameter, causing the patch to push crud ahead of it and back into the powder chamber.
Instead, you need a jag that is .030" smaller than your bore diameter, and the rings should be stepped back in size from the end back towards the rod, with each ring being about .010" smaller than the one before. You want the cleaning patch( pure cotton, or linen, like flannel) to slide over the lands on the way down the barrel, and blouse out and grab the crud in the grooves and lands as you PULL THE ROD BACK OUT of the barrel.
Between shootings, YOU DO HAVE TO FLUSH the barrel with soap and water, pumping a tight fitting jag and cleaning patch with soap on it in and out of the water, so that you create pressure to clean out the chamber and the flashchannel.
Put the end of the barrel in a bucket, or sink, filled with water, and pump the soap and water in and out of the hole for the nipple, which you remove. The difference in size of your .54 caliber bore, and the small flash channel will cause considerable pressure that will blow away and caked on crud that is blocking the flashchannel.
Don't hesitate to use a wire ( a bent paper clip has been used in a pinch) to clear the flash channel after removing the nipple, on the range, if the gun begins to hiccup!
If you insist on using substitute powders, then order CCI#11 Magnum caps, which are designed to ignite that stuff. Use Black Powder, and your standard #11 caps work just fine.
For the cost of the can of substitute powder, you can get 2 pounds of real black powder. See the Links on the index page to find the powder suppliers. You can order as little as 5 lbs. from Graf and sons and get it delivered to your door for about $12 a pound. Powder, Inc. also does small shipments. You save the most money if you order a case, and split it with an number of other shooters.
The reason these guns are fun to use is BECAUSE they require a little effort to learn how to make them work. Not everyone is willing to do that. When you do take a deer, it is a true trophy that you take extra pride in because you had to work a little harder to get it.
Do check the condition of the nipple. If its peened over on any side, that means the hammer is not hitting the nipple squarely, and you should be grinding away the high spot in the center of the hammer. Put some marking die on the edge of a NEW NIPPLE, lower the hammer to transfer die from the nipple to the face of the hammer. Then, cock the hammer back and look at the transfer. If it is not a 360 dgree circle, and of equal width all around the circle, you have a high spot( or more). Grind the high spot( where the dye is) with a small grinding bit you can buy in a hardware store for dremel tools. You don't have to own a dremel tool, as these can be chucked into a normal electric hand drill, too. Grind and test; Grind and test. Until you get that complete circle of dye on the face of the hammer. Then you can be assured that if there is priming compound in the percusson cap( I always look as I am loading my capper tool) the cap will fire on that nipple. By squaring the hammer to the top of the nipple, you end the uneven peening that caused this problem in the first place. Over time, some nipple will peen down, and need to be replaced. However, the Spit fire, and Hotshot nipples have excellent reputations for long life.
Always have an extra, new, nipple in your range box, so you don't have to run out and try to buy a replacement just before hunting season.