What Causes Inexplicable Weight Gain In Child?

I need advice on my 8 year old daughter, I have taken her to the doctor numerous times, had blood tests done, had her thyroid checked, had metabolism checked, and just about anything else you can think of but my wife and I cannot understand why she continuously gains weight. She is currently 155lbs, she goes to a gym twice a week among playing sports constantly. Ill admit she doesn t have the best eating habit but what kid does. When I was a kid I ate 10 times worse than she does and I was always a skinny kid, I am just very confused about the weight gain she has had over the last 4 years. Would just like some answers is all, thanks.