why is it so hard for me to get a flat stomach?

im an 18 year old male and ive been working out for 2 years at some point i had like almost no fat on my stomach, i do ab excersises btw , i have a quite strong upper body too .well i could see my upper abs but the lower ones only showed when i sucked in. it was like i had a belly filled with air. its like imposibble to get a nice ab area for me . why is this?