Flint hold is sometimes quite dependent on the thickness of the flint, therefore the exact placement of the sharp edge if mounted bevel down. My smoothbore lock prefers some flints bevel up, while other flints need to be mounted bevel down as the striking edge will strike over or at the top of the frizzen with extra-thick flints.
It throws these all, into the pan. I don't know if it is the thickness, or the angles on the top of the frizzen. Since I still am using a lot of Rich's river rocks, I have learned to
put them the way they need to be mounted. Some, I've ground the crown off with a green wheel - but be careful not to overheat the chert or it may crack.