Hmm your saddle is very far forward on the rails there. That's probably why the bag isn't sitting well. My guess it's so you can reach the handlebars OK. You might want to try moving it back and trying to get the underseat bag fitting right. You can get a shorter or steeper angled stem to bring the handlebars closer to you instead of having the seat so far forward. Or it that sounds like too much hassle, return the bag and take your bike to a store and try out some to find one that does work.