Glad he ate for you, but I got some suggestions:

If the ball python is eating mice and its in a 20L (long I assume) this tells me the animal is still young and that means the 20L may be too big for the animal so the animal don't feel safe enough to eat, but still may strike and even kill.

The location of the tank can be a problem too, if the animals cage is in a hi-traffic area, this could stress the animal. I personaly use tubs, most of my animals are in sterilite 1756's (32qt) in a rack system. Some are going to need moved into the CB-70 sized tubs which are 41qt) Granted you only have a single animal (I think I read that right) so a rack is probably out of the question for you. Even so, you can set up a tub for the animal, ditch the heat lamp and use the flexwatt style UTH and a thermostat, you should be good to go, just as long as the room temp stays warm, like 75. The tubs hold heat and humidity pretty good which means good sheds! They are transparent, but not 100% A 1756 may be too big for your animal, it has roughly the same floor space as a 20L but a 15qt shoe box (1754 I think) should work

The prey item could be too big or too small. If its too big, the snake may feel threatend and defend it self, if the prey is too small, the animal just may not be interested because it don't move in such a way it does not trigger a feeding response from a snake.

Also, you can offer your snake rats, and personaly, I would instead of mice. I think rats have more "meat" to them then mice do and when the animal gets older, its easier to feed one rat then multiple mice. A fuzzy rat, or chub (eyes closed) is about the same size as a mouse, but its safer to leave a chubby rat in a cage then an adult mouse.

If you offer food and the animal don't eat or even seem interested, remove the food item, if its a live prey item, freeze it for later and just leave the animal be till the following week.

Hope this helps