Sounds to me like maybe they were pulled from the parents too early. It happens sometimes.

Offer it softer food, and just wait it out.

On a side note, how small is the ball you are feeding? Hopper mice are only for the smallest new hatchlings. My littlest baby, a spider, took adult mice at only 115 grams. He's a whopping 400 grams+ now. Nothing wrong with his appetite.

Gale