A shifting flint is primarily caused by a flaw in the lock geometry but can also be the results of something simple like jaws that are too-smooth or leather that is too-hard. Thin and pliable suede leather is best. Lead and bare-backing flints should be avoided as this will cause excessive stress that will destroy the lock. The traditional, and still the best, cure for too-smooth jaws is to rough them up by lifting hooks from the surface of the steel with a graver. If you're not up for that, the traditional field repair, and still second best yet today, is to apply a thin layer of pine sap to the jaws. Southern longleaf sap is the best; sap should be fresh-drawn from a thin-skinned branch as close to new growth as possible.