How do I get my goldfish to eat?

I have two goldfish that I bought about a week apart from PetSmart. They seem pretty healthy, but there is one problem. For some weird reason it's like they don't know how to get to the top of the tank to eat. And you can tell that they are hungry, cause they are digging in the rocks like crazy and eating any little remnants they find, but it's not enough. How do I get them to eat from the top? Also the little one, Myleigh, has an ammonia burn right now, is that stopping her from eating? Please give me an answer soon. I'm worried that they'll starve.