Flat-chested and 15 years old?

Yep basically. Why am I still flat chested when I'll be turing 16 in like a few months? I don't mind being at least a 36A cup like my friend who looks more fuller than me. Also I am not petite. People usually say thats the reason, but I'm far from petite. I come from a family that, lets just say aren't "fit". I'm not that skinny but I'm not fat either. (Well thats what people say, I think I'm still alittle fat). Anywayz, Why am i still flat-chested? It's sad to say that when i look for a dress or bathing suit, everything is fine but my chest area is just not filling it iin. I mean is there something wrong with me? ugh it gets me so frustrated :(