You could try using smaller rats. My snakes tend to be more enthusiastic if the food is smaller. Once you've got them eating smaller rats, work your way up to bigger ones.

You could also try scenting the rats with mice; considering the fact that you feed live this may be a little harder for you. Try putting the rat into a cage/box that was dirtied by a mouse before you offer it to the snake.

Making a transition to a different food item would be much easier during the summer, too. BPs tend to be more picky during the winter, sometimes even young ones. If you get impatient, there's no harm in feeding mice until the weather warms up, and even then she might never catch on. Some animals live their whole lives on mice without problems. Good luck!