Like Melindaste said, we need a bit more information to help you more.
First off, if the snake looks like it's obviously losing weight, take it to a vet. They can check for parasites or some other cause for the weight loss.
Second:
What are your temps? Don't just say "they're fine", humor us and tell us what they are
If you don't know I recommend you do what you need to do to find out, because that's an essential part of keeping reptiles. How big is the cage? How old is the ball python? How big is the ball python? Where did you get it from, and do you know if it's captive bred, wild caught, etc? Do you know what the store was feeding it before you got it? Does the snake have multiple hides, one hide, no hides? Big water bowl, little bowl, no bowl?
And, most importantly:
Have you been handling the snake at all? If so, when, and for how long?
-----
"The python has, and I fib no fibs,
318 pairs of ribs.
In stating this I place reliance
On a seance with one who died for science
This figure is sworn to and attested,
He counted them while being digested."
~Ogden Nash
jenny.thegreenes.org