Ball Pythons are very sooky snakes, I can't stand the bloody things and am glad we can't have them in Australia.
You can either try gradually increasing the size of the feed, or just offer nothing for about 3 months and see how enthusiastic it is to eat whatever you finally offer it. While living abroad (I have to point that out or some here will flip out) I bought myself 1.2 Ball Pythons (which I quickly regretted). One of the females was a complete bitch and was too scared to eat, I spent a couple of months frigging around with her and only got her to eat two or three times, then left her for about 3 months without seeing anything to eat, and when I offered her a mouse after that she was very eager and was always the best feeder from then on.
The same thing works with most pythons and snakes in general. Kids too*
*Use shorter time frames for children.