CONSISTENCY!
It's very easy to put on a thin layer of body fat which can cover your abs. You will still look in good shape, but most of your abs, and your hard work in the gym, will be covered by that layer of body fat. How do you uncover your abs? by being consistent with your nutrition. Having 4 'good days' and 3 'bad days' a week is not good enough to have a solid 6-pack. You need to be consistent every day. You can have 'cheat meals' and the odd bit of 'bad nutrition', but if the majority (90% +) of the time you're good with your nutrition then you will be able to have a good set of abs (if you train hard too, obviously).
It's very easy to look at a fitness magazine and think it's easy to have a core like those models, but in reality, it's not. It takes a lot of hard work in the gym, and being consistent in the kitchen. Monitoring your carb intake as well fat and overall calories is key. Taking on too many carbs will get stored as fat. Consuming too many calories altogether will put your diet into a calorie surplus - which will gain mass, whereas you want to be in a slight calorie deficit or equal calories to what you burn (if you have a 6-pack and want to maintain it)
For a lot of people, the stomach area loves to cling onto fat, which just makes it harder to get rid of. This means that you simply have to work harder. Train harder in the gym and be strict with your diet. A lot of people who have a good set of abs will claim that it's easy (a slight lie..), whereas others will tell you how hard it is and how how much work they put in.
There's no shortcut to having a 6-pack, you have to work your butt off and watch what you consume. Yes it takes a lot of effort, but the result is having a core like those models on the front covers of magazines - surely it's worth the effort?
Lee Gregory Fitness