Pacing the flint with bevel down holds the edge too high and it hits the frizzen too square.....TC locks operate best with black English flints, bevel up, with the following set-up:

Close the frizzen, set the hammer at half cock,and clamp the flint in position so it's edge is barely just not touching the frizzen face...literally allbut touching the frizzen.

In this position you should be able to see that the flint will impact the frizzen at a slightly downward shaving angle...to sition so it's edge is barely just not touching the frizzen face...literally allbut touching the frizzen.

In this position you should be able to see that the flint will impact the frizzen at a slightly downward shaving angle...to shave off the bits of steel...if it appears that it will be hitting the frizzen too "straight on", too "square", then it won't shave/won't spark good, will dull quickly requiring frequent knapping,and often will cause the flint to shatterfrom the straight on, head on impact.

If it's not obvious that the flint will be hitting at a downward angle, then loosen the jaws and slide the flint back away from the frizzen 1/16" (or 1/8") so that when fired, the arc of the hammer will be further forward when the flint hits the frizzen causing it to be lower at a steeper angle.

TC locks with 3/4" x 7/8" BEFs set up as I mentioned above, normally get through a 50 shot session without knapping, or with only one brief knapping.....and oftenfinish two 50 shot sessions on the same flint, knapping 2-3 times during the second 50 shot session.