An Acavallo Gel pad is the best you can get in my opinion it sits perfect and absorbs shock and the the normal one doesn't change to fit !!!
You could try a back raiser
Ah thanks Ally. I'll have a look at that.
I do the girth up like that. He currently has a square numnah under the saddle.
My jumping saddle does the same, I put a thin cut back gel pad so that it doesn't effect the withers but fills in the small gap between the saddle and their back!!
Try doing up the girth on the left and middle strap, sorry if this doesn't seem to make sense it's hard to explain on here ahah. But it should make the saddle sit a bit better on the back, if not, u could try a nummah?