I forgot to add, if the rub marks are on both sides, that's a sign on side to side movement. Try putting the saddle directly on her back with the girth attached and then see if you can rock it easily from side to side. It will move a little, but if it's really pronounced, then there's your problem. I had a similar problem that was solved by using a narrower gullet.
Thanks! I'll try that tonight before I ride...and I do have a narrower gullet I could try (thankfully since it has the Easy Change system if that's the only issue I can just go buy another gullet for $20!!) The one I use now is Medium Wide (she was a Wide when I first got her but she's lost weight and toned up since then! She was really overweight before...), but I'm wondering if she may just be a Medium now. The Medium Wide seems to fit her fine though, it's not sitting on her spine or on her withers...although she has practically NO withers so maybe that's a little deceiving?
Ugh, I need to break down and buy one of those Wintec Easy Change Gullet measuring systems and see what size she is really supposed to wear according to their measurements!
"The times when you have seen only one set of footprints in the sand, is when I carried you..."