I used to have such a flat ass. And it sucked. I wasn't about to get ass injections, so for the last year I made it my mission to grow my peach. The natural way, through good ol' exercise. Here is what I found helped me grow my peach. Check it. When I first started my mission I was doing booty exercises without weights, I got no gains. It wasn't until I started using ankle weights and a booty band that I got results. Along with the weights and a booty band I found this girl on youtube. She really explains how to have proper form, and her workouts are tough.
Hope these suggestions help you grow your peach too.