You're most probably not going deep enough on your squats.. Try sticking your but out on your way down and squeeze your quads at the top of you squat.. Add some weights and stick to a rep range of 10-15...

Also do some step ups on a either a chair or weight bench with weights... If you have access to the glutes machine do donkeys weighted...

There are lots of great workouts besides squats (some are even better then squats). Glute circles (donkey whips), glute combo kickback, glute kickbacks, step ups, lying leg lifts, clams, leg circles, a variety of squats, a variety of lunges, fire hydrants, etc.. You can hold weights for some of these to add extra resistance and put on ankle weights for the others. You also need to make sure that you exercise all of your glute muscles (maximus, medius, and minimus) to have results. If you go to the side bar on this app and scroll to the top, you will see a category called "exercises". Click on that, and select "muscles" at the top. You can find a section called "butt", and it has a large amount of exercises specifically for your glutes. 

Their is only one other way

Brazilian booty workout. 

I am gonna keep this real. If you have a flat butt, all the exercises mentioned are just gonna give you a toned flat butt. These exercises will not change genetics especially if you already have done everything.