Some snakes will just not go to rats.

How did you thaw the rat? Some snakes are particularly picky. Some have had success using a hair dryer to heat up the rat, and then heat the head up to a warmer temperature.

You could try braining. After the rat is thawed, slice into their head and squeeze out some brain juice. I've heard it's very successful, although I couldn't think of being able to do it :p