Take the bolster clean out scew out. Then get a small can and put some solvent in it. Dip your patch and pump that solvent back and fourth through that bolster clean out hole. Also I take a pipe cleaner and dip one of them in a solvent from time to time and push that through the bolster clean out hole all the way into the firing channel if possible. This just helps to make sure it is nice and clean.

After the solvent then run a boiling water and soap bath through the rifle. Be sure and wear leather gloves. After the soap and water bath I like to pour boiling water through the rifle barrel to rinse out anything else that is in there.

Next time you load the rifle ... pop three caps through the nipple before loading. After that run a dry patch down the barrel and remove any fowling caused by the caps. Now push another cap down the barrel and pop one more cap with the patch still down in the breech. Pull that patch and it should show burn marks. If it does, you're ready to load.

When you load, pour your powder, turn the lock down, and slapp it a few times. The tip the rifle up and tap the butt of the rifle on your shoe. This will level the load and also you now should have a little powder in the bolster. You rifle should fire now if the powder is good, the nipple is not some old worn thing, and the caps are quality...

This is what I would do instead of pulling the breech plug. They do make a breech scraper but you can do the same with a .22 or .30 calibercleaning rod and brush. Put a patch on the brush lower it down there and twist in a clockwise motion. Some rifles have an actual cone shape breech and this will normally clean the cone. Don't be surprised if nothing comes out. Water baths really do work.