You may need a different billet system or set up to where it’s not pulling or sliding the girth or the saddle where it shouldn’t go naturally on your horse. This happens quite often and a lot of people don’t even realize it. Check out the saddle fit for life channel or website or Schleese saddlery channel or website. They both have some really great information on this stuff and if you plan on keeping your saddle and this horse together for a quite a long time, it might be worth seeing if you can get it checked and or fixed.