Why does my stomach still stick out no matter what i do???

ok this might seem kind of weird. but i am abt 140 lbs, i have been working on losing weight, & i do a ton of crunches, i try many different ab exercises, you name it! i have tried like everything to flatten the lower part of my stomach, and it still sticks out! i know that you are supposed to work on your posture, and i do that! i dont slouch my back or anything. i have never seen anyone who has this one part of their stomach that just wont get flat! but mine wont get flat no matter what. i see all these pictures of girls with flat abs in magazines, and my sister has them! but i cant get them. i have heard of people haveing 'natural potbellies' where they cant get flat abs, but does anyone know if that is really true? help! :(