Not Growing

[NewLineReptile]

I got a small clutch of Savannah Monitor's a few months ago and sold one to a friend of mine. The one's i have are growing alright but his is still the same size i told him that he might not be feeding it enough or not given enough Vitamins but he said that he give's it Vitamins every time he feeds him and i looked at his Savannah and it is good and fat and very active
What could be wrong with him?

Brandon