What is going on??

[WingedWolfPsion]

You could clear out the tank, and put the tub inside of it, so that half the tub is over the UTH.

A fuzzy mouse is a bit too small for the snake, but that is the point--being smaller, it is less intimidating. (Actually a crawler mouse is more the size that is needed).
I've had some hatchlings start on these when they wouldn't take a hopper. Just worth a try.
The most important points are to feed the snake in the dark, and not to disturb the snake for the entire day before you offer food. Put in the food, and then leave the snake in peace and quiet.

A fuzzy rat is the right size for a hatchling ball python, but since the snake was eating mice before, mice is what you should offer it now. It's very unlikely to eat anything unfamiliar.

I don't recommend leaving a hopper mouse in with a snake overnight. Hoppers have teeth, and are already eating solid food. A hopper mouse could potentially harm a snake. In my experience, if the prey isn't eaten within a couple of hours, it isn't going to be eaten, and should be removed. Wait a couple of days before offering food again.
Snakes seem to have this odd mental quirk--if a rodent is left in with them, and they refuse to eat it, many times they will never eat it unless it is removed for some time and then reintroduced later. This is where you get those stories of a snake 'befriending' a rodent that lives in their cage with them, and the snake only eating other rodents, but never the one that lives with them. People misinterpret the situation, naturally enough, and say they are friends, or that the snake has a pet mouse/hamster/whatever.

Far from being gentle seed-eaters, mice and rats are ferocious omnivores. If left in a cage without food, they can and have literally eaten snakes alive. Snakes will generally not defend themselves from an aggressive rodent--they just aren't hardwired to be able to handle it. In the wild, they would simply flee. So, never leave a live rodent with teeth in a snake's cage unsupervised. The larger and older the rodent, the more serious a danger it poses.