Quite often, when you buy fish, especially highly string ones (eg, tailed black sharks, which have a "rep", Discus & Oscars that "sulk"), they can at times take a little while to get used to your tank.
I'd suggest leaving all the lights off for 24 hours, and trying to reduce noise near the tank, and not feeding. (my discus took 2 weeks before they started to come out from behind the plants at the very back of my tank )
Unless the fish is ill, perhaps someone else here can give you a hand...