Geometry of the strike is off. Your flint is hitting it too directly and is smashing the tip of the flint. The flint needs to slice down the frizzen from the time it strikes it. A longer flint will just make it worse. Flipping it over might make it work because it may hit the frizzen at a different angle. The real trick is to get the flint to strike the frizzen at the point of contact at about a 55° to 61° angle. If it does that it will slice a lot of sparks off the face of the frizzen and last a very long time - sometimes over 100 strikes.
This is a very much a "Goldilocks" type of situation, so feel free to experiment with it a bit. If the point of your flint hits it at too low of an angle it will smash the edge of the flint or break it. If it is a too high of an angle, when it hits the frizzen it will knock a big chunk of flint off of the bottom of the flint. It knocks that off because it's literally slapping the top of the flint onto the frizzen instead of having the tip of the flint scrape along it.
Unfortunately the lock on those Traditions longrifles are really small - pistol locks really. My first longrifle was a Traditions Pennsylvania longrifle and it had that same small lock using that 5/8" flint. You may end up needing to get the square 5/8" flints to get the flint back enough to increase the angle. It usually strikes the frizzen closer to 1/2 the way down the face of the frizzen instead of 1/4 of the way down.
From what you are saying I would expect that you need to slide the flint back in the jaws to get a steeper angle on the strike. If you're using leather to hold your flint in the jaws, you may need to put a notch in the middle of the leather so you can slide it farther back against the jaw screw. To do that, take you piece of leather; fold it in half; and then cut a V notch out of the middle of the folded over part with your knife. That will allow the flint to slide beck further if you need it to.
Once again, this is a bit of the goldilocks effect, too little of an angle=smashed tip of the flint; too much of an angle will knock a big flake out of the bottom of your flint. Just right and the flint will be self sharpening and seem to almost never wear out. Use whatever that last angle turns out to be on your lock. When you find that exact angle on yours, get a good look at it or even sketch and put it in your shooting bag or box. Refer to it as needed.
One of the other drawbacks on that lock is that the pan is very small and can be pretty picky how full you fill it to get it to reliably ignite the main charge. Mine worked best when I filled it about 1/2 full, closed the frizzen and then holding the rifle by its wrist I would quickly twist my wrist with a snapping motion to the right.
That would actually bank the powder in the pan away from the flash hole in the barrel leaving a little empty space close to the barrel. Theory is that left an empty flash channel on that small pan for the flash to travel into and through the vent. Whatever it was, my Traditions longrifle went of first time every time as long as I did that. Certainly, where the flash hole is located in relation to the pan has something to do with that and that location on Traditions rifles doesn't seem to be too standard.
The lock on my other longrifle is a L&R Queen Anne lock and it has a pan that is easily 3 or 4 times the size of the Traditions pan. It doesn't care at all how much or how little powder you put in it, or whether it is banked to the right or not. It just goes off first time every time, all the time.
Hope that helps.
