A lot of times they're mostly starved when you get them. They require special feeding attention, which they don't get at a lfs chain store.
When you get one, I think it's helpful to keep them in a smallish tank until you have them fattened up. They're not supposed to look all bony like most of them do at the store. Feed them with a turkey baster. Once they get bigger they kind of catch on a bit better as to how to go after the food.
You should try to get your money back since it died so soon after purchase.