Remember, don't just thaw them out. The best way to trick them is to make them as warm as a living one would be. Ball pythons hunt by heat, so if it's cold at all, it will likely get refused. I use a blow dryer to heat up the heads of the rats a little more to give a good target for the snake. The most common thing my snakes go for if I don't warm it up properly is a foot and they try to swallow the rat by the shoulder. Usually they end up firguring it out but a couple times they've given up and I had to zap them again with a blow dryer to get them to eat it.