If you haven't given the snake at least a week to settle in (handling during the first week in its new home), that could be a possibility.

Temps and humidity are very important to have spot on because most Ball pythons will refuse when temps or humidity isn't correct. And it would also be very important to have the correct temps for proper digestion of the rodent fed to the snake. What are you using to measure the temps and humidity, and what are you using for a heat source?

Some Ball pythons are picky when it comes to certain rodents. Some will eat colored rodents, some will only eat white rodents. Some prefer mice over rats, and some prefer rats over mice. It really depends on what they have been fed previously, so if it was fed rats previously but you're offering mice, that could be a possibility on why the snake is refusing.

Hiding places are also very important. Some Ball pythons will refuse meals if they don't have a secure hide. They like to be hugged on all sides, including the top. So, purchasing a tight-fitting hide (not too small to where the hide rises or the snake's body is sticking out from underneath the hide) would be the best to do. Having a half log hide wouldn't do, they are not secure for Ball pythons at all.