My paint gelding has pretty high withers, he rides in a Semi-QH bars saddle. I have a lot of problems keeping the saddle from sliding back to where the pommel is on his back, and I can see his entire wither(s) when I'm on him. I use a breast collar, and it helps somewhat, but it still slides back a lot. If I made it tight enough to where it would stay in place, he wouldn't be able to breathe when he lopes.
Maybe I'm not doing something right, or I'm wrong about were I thought it should go, or something's not right, but there has to be a solution to this problem.