If you find a good source for gun-flints, buy more than you think you'll need. It may be a while before you can find quality flints again. Tightening the jaws of the cock directly onto the flint will rarely work, as the grip is poor. To improve the grip, something needs to be wrapped around the flint before it is inserted. Either leather or thin sheet lead can be used.
Most shooters find leather works best, as lead tends to compress, loosening the flint.
Wrap the leather around the bottom, back and top of the flint, insert it into the cock and tighten down moderately. Move the cock and frizzen so the cutting edge of the flint is touching the frizzen, and is flush all across the frizzen face. Hold everything firmly right there and tighten the flint down well. If the flint needs to be adjusted back closer to the cock, cut a small hole out of the leather so the flint is directly touching the post of the cock jaw. If that's not enough, break off part of the back edge of the flint.