Welcome to the world of rock and steel. Flints shatter at different rates based on the angle of the fizzen, the angle of the hammer, the angle of the cut on the flint, pressure of the springs, and event internal fissures within the rock. Make sure the flint is not striking the fizzen at too direct an angle (i.e. stabbing into it) but is actually scraping down it to slice of some steel thus creating sparks. The flint also shuld be held tightly in a padded material such as rahide or better, lead. This transfers some of the shock away from the rock where if it were contacting the steel of the hammer holding it and the steel of the fizzen it is being driven into, the rock will more likely shatter since the vibrations have to be relieved through the weaker part rather than stronger parts.
My own KY flint lasts for scores of strikes, probably 40-60 with machined flint. A 1757 Tower horse pistol I have using big English flint has lasted perhaps 20-30 shots I have fired it for, but my Brown bess will shatter her big English flints about every 15-20 shots so regularly that I know before going into a battle with her to have a spare flint and the tool in the front pocket of the cartridge box. After 10 or so shots if we are pulled out briefly I usually take a minute to swap rocks just to be ready since I know it is coming.