I'm new to English, and taking lessons (so far I've only taken one...) on a horse that's not mine, but I also like to practice riding English at home with Specs. I have a Silver Fox AP saddle that's really comfortable, but it tends to ride up really high on his withers/back, when I'd like it to be back farther, if I'm making sense. My trainer said my saddle fits him well, it just tends to slip up. At the moment, I'm riding in a crappy rope-type girth with a crappy old saddle pad that's at least 30 years old that I found in an old tack box (and it's very thin.)
I was thinking that it was the girth's fault? Or maybe I'm just not used to looking at an English saddle on him and it just looks different than a Western saddle? Idk.. Here's a pic - I think it's too far up... but maybe not...
I suggest you take front, back, side, and far away views of the saddle just laying on her back and girthed up (without pad) and girthed up with pad in good lighting. That'll help you see what is going on exactly.
It could be a number of things
Scenario a) it doesn't fit as well as one hoped
B) the pad isn't right for the horse
C) it could be the girth
D) she could need a half pad/correction pad
E) it was a mixture of some or all of the above