My first ball python would not take f/t rats for me either. He started on the mice, but I really wanted him switched over so I wouldn't have to keep buying and then feeding so many mice in one sitting! It took a lot of work and was very frustrating, but I finally got him to accept!
What I did was just refuse to feed for a lil while. I made sure he was nice and hungry and then I dethawed a nice big rat for him. At first he still wanted nothing to do with it :/, but upon reading things on here I think I got him to eat because one special thing.
Obviously mice are smaller and keep the heat retained in them better right after being dethawed. When I offered the rat I think it was still way too cold for his liking and thus he didn't read it as being something to eat. I now use a heater(or could use a hair dryer held close) after they are thawed to dry them and keep them warm. I especially focus on the head area so when I dangle the rat it's right there all nice and toasty. He takes them no problem now and comes rushing from his hide when he smells me approaching with food. Maybe you should try getting them RLY warm (without overdoing it) and see if anything changes? That's all I know how to do and would be worth a shot if u haven't tried already!