Adding muscle, but loosing the six pack? What am I doing wrong?

Anonymous
I still am a bit underweight for my height.
So I started to try and eat a bit more and do some workouts. I have dumbells, a pull up bar, elastic bands or whatever they are called.
My shoulders got broader and now are more muscly, but seems that the six pack started to fade away.
So I searched for a routine for abs on internet. It consists of sit ups, crunches, leg raises, planks. And I can really feel the burn and the exercises get tougher by day, but the six pack is not coming back for the moment :)
What the hell happened and what to do toget it back?
Adding muscle, but loosing the six pack? What am I doing wrong?
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