If flintlocks were so great, why did they give way to cap locks and eventually center fire?
Maybe not a reasonable argument but one thing for sure flintlocks are way more fun! 
I shoot everything and throughly enjoy shooting my flintlocks but when the rubber meets the road caps win. 
Everything has its pluses and minuses. One must remember that the flintlock never went out of use in America. I think it was Cline who once bought an original rifle from an old man he had heard had one and when he arrived the man was preparing to load it to go Squirrel hunting. The same can be said of percussion. But its only been in use since the 1820s-30s. The caplock was a revelation to wing shooters in England due to lock time but the FL held on longer in rifles according to George. There were people who went to CA in the 1840s with a flintlock rifle because percussions were not really trusted. In the early 1830s the American Fur Company SPECIFICALLY ordered flintlocks from Henry stating the percussion would not do.
Moderns, especially people who have never had much contact with GOOD flintlocks seem to think that the flintlock was immediately thrown on the trash heap about 1830 because "percussion is better". But we need to remember that OUR percussion system is not THEIR system. Horridly corrosive caps, inconsistent caps. Poor nipple design, blown barrels of gun converted to percussion from flint. No it was not "better" in all respects.
Here is the deal. Percussion guns have a lot of hidden passages. They don't do well with massive holes in the nipple, accuracy wise, and they are prone to problems. Wiping between shots is more likely to cause a misfire with a percussion than a flint since the flint has a simpler flash channel.
With a flintlock if the vent is clear and the flint sharp and primed PROPERLY it will fire. The problem is generally with the operator if a flintlock misses fire or flashes.
And YES in some places its possible to pick up "rocks" off the ground that will work. I have done it. In my area its Agate generally. Now if in the west and you find even one it will keep you fed for a long time weeks maybe. If the person is low on flints and REALLY looked for something he will find a few more. Shoot a buffalo jerk the meat and it will run one man for quite some time. I picked up 2-3, one lock ready as it came from the ground, walking 1/2 mile back to my truck one day.
I have a friend with a percussion rifle I built some years back. In matches he wipes every shot and has to pop a cap before loading. This rifle has a good breech and I taper reamed the flash channel to almost the nipple seat to assure it would not have problems. It does. Why? I have no idea unless its something the breech maker did that I missed.
Why do flintlocks fail to fire? Poor design (modern usually but some "vintage" locks are $#@* too), poorly hardened/wrong steel alloy frizzens, not knowing how to run a flintlock, having wet fouling in the pan and/or vent. Believing that a little priming is enough. Letting the flint get dull, one of my failings at matches. There is a long list.
Caplocks with poorly aligned hammers, poor nipple design, poorly laid out flash channels. There is a list here as well. Not to mention that the percussion system is more prone to accidental discharges after the cap is removed to make it "safe". Its better to leave the cap on.
A good flintlock gives little or nothing in reliability to the percussion. But the percussion is easier to shoot, for most people. If properly made and the caps are right its a little more reliable in the wet unless the FL shooter has a cover. Many percussion shooters get into trouble by thinking it does not need to be covered in wet weather.
Yeah I have run around the mountains guiding hunters on pretty wet 10 day go arounds and had no problems. Good lock, good management FL works. I have difficulty building percussion guns. I just don't like them. Some bad experiences years ago that were worse than a misfire and a few others related by friends and others.
As I have likely posted before. As late as the 1840s experienced western hunters were still telling people that the percussion was not reliable.
People need to shoot what they like. But coming to a site inhabited with flintlock shooters, many or whom are well experienced and KNOW how to make the "run" well and have done so for decades and spout off about people having trouble with them. I have seen people REPEATEDLY jam my absolutely reliable 1911 Colt because they did not know how to shoot it properly. They limp wristed the pistol which causes it not to cycle properly.
Buy a Wall Mart FL, put 1-2 gr of priming in the pan and it will start to flash in the pan if not initially in a few shots. Buy a recreation of a Manton FL put it into a properly designed and assembled gun and use the proper level of prime, a good flint and its going to fire. Same for a Chambers Siler etc.
Cheap flintlocks were junk in 1770, in 1830 and they are junk now. One of the American Fur Company's instructions to Henry was to take great care in selecting the locks.
Dan
