Several problems may exist or have arisen. First, are you using genuine flints? The real thing works ever so much better than the sawn blocks of agate that usually come with these rifles. My favorite flint source is Track of the Wolf.
Flints can be mounted with the bevel- the slanted face- up or down. Try mounting with the bevel the opposite of how it came from the carton. You want the edge of the flint to strike the frizzen at a fairly steep glancing angle. It needs to scrape off bits of white hot metal, not bash head on into the frizzen. Experiment a little with a leather pad under the flint to raise the strike point a little.
Getting the frizzen to flip open is important. Buy or find someone with a spring vise, an important tool for gunlock tinkerers. Improvised tools break springs. As said above, polish the area on the spring where the lug on the bottom of the frizzen bears mirror bright with finer and finer grades of emery paper. Polish the bottom of the lug the same way. Keep this area clean and lubricated. I like to use chain saw bar oil here- sticky and designed for sliding contact- but any good oil will work fine. While polishing, smooth up the sides of the frizzen around the pivot hole, and the ears on the lock plate.
On many of these mass manufactured locks it is necessary to remove a little metal from the front of the lug, thus moving the apex or tip over point to the rear. This is fussy- get help from a local expert.
White Fox