My snakes will do that if the rat is too big. You should weigh your snake and weigh the rat, should be around 10% of the snake at most. I'd try inside the enclosure with a smaller rat or an adult mouse. My big 1500 gram girls will often take a fresh killed mouse when they are in a fast. Or if you prefer live I'd try a rat pup that still has its eyes closed, usually they get bigger than jumbo mice before they open their eyes.