I agree that you'll probably just have to get a better fitting saddle. Truthfully, the biggest problem I see is that the saddle appears to long for her back. Couple that with her slight downhill build and the less than perfect fit of the bars and it's slipping forward. Unfortunately, I think all you would accomplish by having the rigging moved would be to start wearing sores in her girth area.
Corriente makes a good saddle for broad backed horses, but they are more working quality saddles, not so much show quality.