I could be way off as Im not good at determing what causes stuff with leather alot of the time but it looks like you could be beveling when the leather is still a little to wet.
Take a piece of your leather your using and case it like you would, cut a line with the swivel knife and bevel that line, enough to get a good example, then let it dry some more, come back and repeat. Do this a few times as the leather dries and see if it improves your beveling. If you find a spot where it seems to smooth out easier then you know you found the right dampness to tool it.
I know when I first started I allways started tooling to early. The leather would mound up on the edges of the beveler and was a pain to lay down because there was too much water in the leather. I gradually learned to hold off longer till I started and it made a world of difference.
Good leather helps too
Letters look really good though.