Failure To Thrive

Well my little princess has been diagnosed and now they are trying to test her for many things. Her last visit she was seven months and weighed 12.4 lbs and has completly fell off the charts. We know she is tiny but she eats well and seems healthy to us. So my question is pretty general I want info on Failure To Thrive. I did search the internet and it was all pretty general by saying its a calorie problem. So anyone have some first hand experience with this?