Do you have the UTH attached to a thermostat? They can get hot enough to burn the snake if uncontrolled, and BPs will sometimes dig under the substrate to get to the glass where it's hottest. So a thermostat is really very necessary. A good beginner model is a Hydrofarm/Jumpstart from Amazon---about $30. Don't use the UTH until it's controlled; you really don't want to risk a burn.
Baby BPs sometimes feel insecure in a large enclosure. Open spaces make them nervous. If she won't eat or otherwise seems anxious, you might have to clutter the tank up some more (silk plants are good) or even move her to a smaller enclosure until she gets bigger. Some babies are OK in a large tank though, never know until you try.