1. YOU HAVEN'T BUILT THEM- Like any other muscle abdominal muscles can be built to be bigger and stronger. A low body fat percentage isn't always enough if you are lacking abdominal muscle structure. Strengthen this muscle group by hitting the big compounds consistently (deadlift, bench, military press, squat, pull ups etc).
2. YOU EAT TOO MANY CARBS- If you are constantly fuelling up with carbs, your body has zero reason to get savvy at using fat as a fuel source. Only when carbs are low enough will your body begin to tap into that stubborn stomach fat. Consider lowering your carbohydrate intake and bumping up protein consumption.
3. WEEKDAY WARRIOR, WEEKEND WASTER- You keep structure and discipline during the week with protein rich breakfasts and regimented workouts but you take your foot completely off the gas on weekends. You're over consuming alcohol, not bothering with training and making poor nutritional choices. This yoyo of a week means progress is never made. I always want my guys to be able to ease up on the weekend, perhaps go for a drink or a nice meal out BUT there is a difference between 'easing up' and going full on self destruct!