Defrost and warm the mouse and the rat together. A little mouse blood on the rat seems to help. Try to use a rat the same size and color as the mouse. This has worked for me. I have converted about five animals. If the snake is loosing weight I will sometimes feed the mouse that was defrosted with the rat. Gets them used to the change of smell. Once the snake finally takes a rat I never allow them to eat, see or smell a mouse again.