A few questions first DocHunter.

Are you using lead or leather for the flint wrap?
How many stirkes did the lost flint have before it disappeared?

It isn't unusual for me to have a new flint flake a bit on the first shot. I just chalk it up to the flint fitting itself to to frizzen.

I use lead to mount my flints in the cock, and always have to retighten the cockscrew after the first shot, and usually after the third or fourth shot, because the lead is forming itself to the flint with the first few strikes. After a few shots it locks itself in. But I still check the cockscrew after every half dozen shots or so.

In my Great Plains flinter the most common cause for a failure of the strike to ignite the pan is a loose flint.