Foraging is a learned behavior. They have to learn what things are food, what things are safe to eat and what tastes good. Without a mother hen and other members of a flock to show them, they only have trial and error to teach them. If you have a bold chick in the group, they'll often try things first and show the others. If they're all shy, it'll take longer.
If it's a food you would normally eat, try eating some of it in front of the chicks. You can also hold a food in your hand, look at it, rotate it, tap it with a finger and then taste it, while they watch. You can also cut things into smaller bits at first, to make it easier to eat. If there's a food they already like, even if it's only their chicken feed, try sprinkling some of that on top of the other food. Once they start eating other foods, they usually become more accepting of new foods. Sometimes it just takes time to get them started.
