Which store to buy a Ball from?
I've seen BPs get scared by people moving mice around in an effort to get the snakes attention. If you've done that, chances are thats what may be turning him off to the idea of taking a strike at it.
More likely though, its in their nature to go off feed for periods of time. Just keep offering prey items every 5-7 days, and if he's hungry, he'll take one.
Don't worry about it until he starts to show outward signs of ill health. When and if that happens, you will need to consult a vet and get him checked out for parasites or other health ailments. It IS possible that he picked up a bug from one of the mice that he ate, which is now causing him to go off feed. I'm not saying this is definitely what happened, but it is something that you should be aware of.