Try different flint positions, and bevel up or down.
You know the leather that wraps around the flint to hold it in place? Leave it long on the topside so the frizzen bouncing back doesn't smash the flint.
I've found on one particular lock that I had to put bevel UP, which said flint hit the frizzen lower down, and did not slam it open as hard. This resulted in a more shearing action, which still made yummy sparks.
While testing your lock, do this in dim light so that you can see where the sparks go. Each position will cause the sparks to land in a different spot. Ideally, they will land smack in the prime.