Your abs are just like any muscle, to build them, you have to break them down, use resistance, ie: weights on crunches, machines with weights, etc. just plain crunches, sit ups, etc work, but to get bigger abs, have to use weights. that may or may not be your problem. ill do a search on pics of you and see how small you are. you may just have really small abs and need to make them bigger..?
Topical Yohimbine CAN help in losing the fat over your abs, especially love handles(though diet and exercise are as always very important).
As far as being able to see your abs even when less lean.... training them heavy (though with good form and support) is the best way to increase thickness and protrusion of individual "abs"(technically incorrect but as far appearance true) (stretching is also very important)
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