Things to consider
1. Is the snake about to shed?
2. Is the snake depressed?
3. Is the humidity at appropriate levels
4. Is it the mouse color, some snakes won't eat white mice.
5. Whens the last time the snake had a stool?
Suggestions.
1. Put the mouse in warm water to stimulate their hunger, it works
sometimes.
2. Always feed your snakes on the same days and times.
3. Don't feed it when it's about to shed.
4. The snake may not be hungry and refuses to eat.
This link will help you a lot
http://www.kingsnake.com/ballpythonguide/