What have you been trying to feed the little one? If you have been offering F/T maybe try live. If you have been trying rats try mice. Also don't worry if they haven't eaten in 3 weeks because ball pythons tend to be a picky bunch. I also would suggest keeping the feeding attempts to once a week so that the baby has time to relax from the last attempt.
Also what are the temps for the hot and cool sides? How have you been regulating the temps? Most petsmart employees are trained to sell you the "package" they offer which is usually not best for a ball python.