Some snakes are hard to transition over once they get started on a particular prey item. Your snake is young so keep trying to switch it over, if you can get live rat pups that might help move things along.

However some snakes are just picky and will only take mice, even adult snakes. If this is the case you will have to feed multiple mice to the animal to ensure it's getting the necessary amount of food.

Good luck, keep at it sometimes it just takes some time for things to click.