Try rat fuzzies 2 to 3 times, and if that doesn't work, then try mouse hoppers. Make sure the baby has an appropriate hide. Feed at night, at least a half hour after the lights go out. Don't hover--toss the food in with minimal fuss, and leave. Do not uncover the hatchlings if they are hiding.
If the hatchling still won't eat after 3 weeks after its shed, fill the bin with crumpled paper, wait a day, and try again with a mouse crawler. If that fails, assist-feeding is in order.
Be prepared to assist-feed if you get a holdout that simply will not eat. Generally after one or two assist-feeds, the hatchling will begin taking food on its own. Seems some of them need a 'jump-start'. It's rare--exhaust other methods first. Do not, however, wait so long that the hatchling begins to look too thin. Assist-feeding is stressful, and if it has to be done, it should be done while the hatchling is still strong and healthy.