When you’re training for anything always remember the SAID principle – specific adaptation to imposed demands. Your body will adapt to the demands you place on it, and it doesn’t care whether that gives you a six-pack or not.
Your body doesn’t care how much you sweat, how hard you’ve worked, or how sore you are, unless you’re sending it the right messages.
Just like any other muscle the abdominal muscles need purposeful training and progression.
If you’re just aimlessly chucking in a few sets of abs at the end of your workout, and hammering the cardio, check out this TFM article written for Breaking Muscle.