She's probably just getting used to the surroundings. Fish have to get accustomed to the tanks. My clowns and firefish did the same things; hid behind rocks all day, for about 3 days. Then they all started coming out and playing around in the entire tank. About the food you feed it, what kind of food is it? Sometimes fish are picky and like specific foods. When you were at the LFS
did you see it eat? It's always wise to ask them to feed the fish so you at least know what they are feeding them, and that they aren't sick. I would try a different food if its not eating the flakes. Something like freeze dried or prepared foods, or maybe mysis shrimp. A lot of fish love mysis shrimp. Frozen or not.