Hello everyone

First time poster, and I come with a question.
I recently ordered my first binders ever, decided to go with Loveboat because I was worried about comfort more than flatness. I'm a bigger chested person, with the widest part of my chest being 42 in/107 cm, so I didn't really have high hopes to become completely flat but I was hoping to at least get a decent result without killing my ribcage.
Ended up ordering their 2nd Gen Jersey Mesh Short (size XL) and High Performance Velcro Short (size 3XL), both had really good reviews and I was careful picking the right sizes following the site's charts. I wondered why it was 3XL for one and XL for the other, but at first I only thought that being different models the sizing was different. However...
As soon as I got the binders in the mail, I immediately realized that with my chest shape, it would have been very difficult to bind effectively with a binder that doesn't stretch. My problem is that my breasts are naturally quite low, and saggy, so no matter how I position them inside the binder, after a while they end up slipping out from under the binder. With very uncomfortable results.
I was able to solve the problem with the High Performance by wearing a bra underneath it. Keeps everything in place and the binder still binds good (as good as it can with a person my size, I guess. Going from a triple D to an A or even B cup is a great conquest for me). I honestly love this binder and will likely get another like this in future.
But the 2nd Gen Jersey Mesh just won't keep
anything in place. It doesn't FEEL TIGHT per se and I can close the velcro just fine, so I guess the sizing is not wrong, but after a few minutes of wearing it, it slowly rolls up exposing the lower part of my chest and so it's like not wearing a binder at all. I blame it on the fact it doesn't stretch, unlike the High Performance.
Before I call it a loss and I sell/donate this binder, is there something I can do to fix this? As I said these are my first binders ever, so it's highly possible that I'm just doing something wrong when wearing them/repositioning my "stuff" inside them. I am using a regular bra underneath them, if this changes anything (I don't own any sports bra).
If needed I can provide pictures.