Why haven't I lost my tummy?

In April 2013, I decided I wanted to lose weight and get healthy. I lost 50 pounds and went from a size 12 dress size to a size 2-4. I even lost some of my thunder thighs. What I haven't lost is my tummy. Its much flatter now but I wouldn't say its tight or abs worthy. My diet is healthy so why hasn't my stomach slimmed down ?
Why haven't I lost my tummy?
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