My saddle fits my horse well except in this aspect, but the other way, the tree wants the withers to end further back than they do, so the saddle shifts forward. For me its gotten better as he has muscled up, but the best thing I've found is the ecogold secure pads. They are pricey but absorb most little discrepancies between the saddle and the back because of memory foam.
You should also try a kn saddle if you can get your hands on one..
Posted via Mobile Device