Even the best fitting saddle in the world will often pop in the back when a horse puts their head all the way down because it causes their back to flatten out.
Like others said, it's hard to judge just based on a few pictures, but it appears to me that the bar angle may still be just a bit too flat for her. In the picture without the pad, it looks like the top of the bars are fitting tighter than the bottoms of the bars. The width looks good, though.
I think your biggest problem with this saddle is the full rigging. I hate saddles with a full rigging, even though I know they have their place. I much prefer one with a 7/8 rigging. Though there are some horses that still need one that is 3/4 or so, I just use the 7/8 on everything because the 3/4 isn't safe or secure for roping.