Something that everyone else has left out is the quality of the lock, and specifically the temper of the frizzen, on your gun. A too-soft frizzen can allow the flint to bite too deeply. This results in good sparks for a few shots but also takes the edge off the flint in a hurry. Then there's also the question of the angle of the flint in the jaws of the cock. At half cock with the frizzen closed, the flint should ideally just kiss the frizzen about 1/3 of the way down the frizzen face. It should also not impact at a 90 degree angle but should be angled downward so that it scrapes instead of acting like a hammer driving a nail. Flint bevel up or bevel down? Everyone seems to have a preference. I think it depends on the flint and the lock and goes back to the factors listed above.
Sure, you can re-grind a flint. But you are going to need a diamond wheel turning at high speed and preferably a water bath to prevent overheating and cracking. Those rigs aren't cheap. Far better to learn to knap. It's really not hard. Here's something to try. Check your flint before every shot. If it is getting dull take a moment to knap a new edge. Just a little. Don't go crazy. Alternate the sides that you knap. If you knapped from the top this time do it from the bottom next time. Take the flint out and flip it over in the jaws to knap if necessary. I'm NOT saying to knap before every shot! Just check before every shot and knap when necessary. It is basic part of shooting a flintlock and you will get a feel for it.
Storm