Why is my hamster lethargic?

I just got my hamster Moo and so far, all he's been doing is sleeping in his cage. He's healthy and has a nice cage with food and water and fluff and a wheel, but all he does is sleep in the corner of the cage. In the store i got him from he was happy and on his wheel, but in his new home, he's not the same. i don't want to give him back but it seems like a bit of a waste of money. PLEASE HELP!!!