This is a situation that can plague anyone shooting a sidelock in particular. I'm just getting into flintlocks right now and have had all sorts of misfires... but since it is probably mostly my own doing till I become more familiar with all of the guns idiosynchracies, I'm going to contain my thoughts only to the caplock.
Lots of things can cause the gun to go click, without the follow up boom. They can be:
1) Improper cleaning
2) Moisture
3) Nipple channel or fire channel not clear
4) Nipple not clear
5) Poorly constructed caps
6) Nipple not forceful enough to deliver the flame
7) Breech is not cleaned up and powder is not inline with the channel
8) Cap not securely on the nipple and flush
9) Too weak a spring
10) Related, too much drag on the hammer with a weak spring
Cleaning: This is important both after shooting and again before shooting. When you are through shooting, make sure you clean your gun thoroughly and then after you have assured yourself of a dry clean barrel and lock area, lightly oil your bore. Before shooting again I would clean your bore thoroughly again with alcohol and dry patches and then run either pipe cleaners or twisted wire with a patch through nipple channel to assure yourself of no oil or other debris.
Personally before loading my gun, and after popping a couple of caps (these can leave debris that can plug the nipple channels too with no blowback) I like to fire off a full charge into the air (no bullet obviously) or into a dirt bank to assure myself of all oils and that the gun will go off. If you cannot do that, then after you have loaded your gun, remove the nipple and drop a few grains of fresh powder down under the nipple.
While loading... MAKE SURE THE BORE IS DRY... Cannot stress this enough. When loading for hunting, some hunters insist on loading with a loading tube so powder doesn't stick to the sides of the bore. This is extreme, but it does accomplish another necessity, it concentrates the powder around the flash hole. One should always turn the lock down and gently tap the breech area to tap powder into the flash channel.
Moisture... If humidity is high, cover both the nipple and bore with either a baloon and a rubber balloon under the hammer (no cap). IF you are going to walk around with the gun capped, try to seal it with blistex,vaseline, or paint it with nail polish (smelly and not first choice)
Poorly constructed caps... Caps that leave a lot of residue can leave your nipple filled with by product that can plug your nipple. Always have a nipple prick that you use as a final loading action when hunting.
Some nipples, particularly some factory nipples, don't deliver a real "shot" or jet of flame to the powder. Some aftermarket nipples like the hotshot and surefire nipples for example are designed to deliver a a jet of flame to the powder. Possibly switching to musket caps would be a good idea too (this is what I have done to even my #11 guns).. lots of flame there for sure.
Remember though, it is the least amount of flame necessary to ignite the powder that will give you the best ignition and accuracy shot to shot.
Sometimes our breech area will get a build up of carbon. This build up is not healthy for your gun. From time to time take a scraper and with your ramrod, try to clean out the breech of impacted by product. Make sure you clean this thoroughly and everything is dry before you load again.
Weak springs are a reality sometimes and changing them out is necessary to get good ignition. Another culprit that is related, is a nipple that is out of round, hammered flat that impedes the cap from fitting on flush. This even with a good spring will require two strikes to set off the cap. Change out the nipple or dress it with a file till the caps fit flush.
Lastly, you can have too much drag with poorly constructed locks. Polishing these areas (the moving parts of the lock and where they come in contact with each other) will produce smoother workings of the lock and a faster hammer throw.
Otherwise I don't do nothin... just go out and shoot...
Aloha...
