Helpful post by Steeltrap.
You can check the angle of contact and how the flint slides down the frizzen easily enough by slowly releasing the cock from halfcock, and letting the flint contact the frizzen and then slide down while you hold everything in your hands, controlling it to make it move slowly while you watch. Wear a pair of gloves to protect your fingers.
You might also try knapping the edge of the cut/sawed flint. Sawed flints hit the frizzen like a garden hoe, evenly across the face. This distributes the force, and minimizes any particular point of contact. That might sound like a good thing, but it's not. It's the opposite of what we want. To the extent that they dig in at all, it will just be where they first hit, and at that spot they will dig a long, horizontal divot across the frizzen, often knocking the frizzen back at that point, without slicing off much in the way of slivers that will be sparks. In contrast, a knapped flint hits the frizzen more like a garden rake, with focused points of pressure, and (in my experience) is more likely to slice off the kind of slivers we are looking for. Knapped flits will wear and chip a little as they are used, of course, and that's like reorganizing the tines of the rake in a random manner. So you get both strong points of contact on individual strikes, and you also get a random wear pattern distributed across the face over time.
(NOTE: I'm not saying that cut flints never work, guys, so please don't flood this thread with a discussion of pros and cons of cut flints. Let it focus on the original question.)
If you want to try knapping it, I'd recommend taking it out of the lock to knap it. No need to risk breaking the lock. Take it out, clamp it (with leather) in a bench vise, and knap the edge very lightly with fine chips all the way across, just like you would any flint. Then put it back in the lock.
It may not help, but it certainly is a place to start.
