They might be too timid to leave the protection of the plate or the air is too cold for them. Chicks can do without food and water for a day or two but I like them to be moving around and acting like chicks as soon as possible. You might have to encourage them by scattering a bit of chick starter around them and using your finger to move the crumbles about. Keep water close to them. Remember, chicks may huddle under the hen for a while before she decides to start foraging. She then talks to the chicks and uses her beak to show them what to eat. Let see a picture of your set up so we can offer suggestions.