If it was eating Gerbils without any problems before You got it, You should try feeding it Gerbils again, maybe throwing the rat in te cage just after it eats the gerbil to see if it eats it too. Remember that changing a Ball Python`s from one owner to another could be a little stressful to the snake, and it may take a little time before she eats again. When I bougth my BP it took me 9 months before she ate here first meal with me, and she hasn`t stop eating since then. Good luck.