How old are you? Why are you worried about your ass so much? In most mens opinion if it is tone and not jiggly with celulite than your doing really good. Just because you don't like the way clothes fit on you doesn't' mean you don't have a nice backside. If you are really young, like less than 24 then just wait, your ass will get bigger in time.
I would make sure you are eating a proper diet. Many people fail to see exercise results because their diet is trash. It's not just a matter of eating healthy, but you have to eat enough calories and get the proper macro nutrients if you want to see any muscle growth. Otherwise, not much you can do unless you want implants.
Get over it. Sure you could exercise but there are just some things in life, especially physical things, that aren't meant for you to achieve and I'm sure you have other attractive attributes.If you and some guy were meant to be lack of chunk in your ass cheeks wouldn't stop it.