If they won't take a f/t rat at first, try live and after a couple of meals, switch them back to f/t. Although, this might backfire on you and have you pulling out your hair.
Case example:

My pastave, Happy (nick named "Stupid")
He started out on live hopper mice but eagerly took them f/t. I tried to switch him to rat pups... scenting every way imaginable. Only managed to get him to take one by hiding a f/t mouse behind it (that only worked once!). The little ****** still preferred mice.. so I fed him mice until 300g and offered a live weaner rat that was housed with mice fuzzies on the trip home. Bam! He ate the darn thing. And started eating live rats. But he won't touch f/t rats... and right now he isn't eating at all. He's my frustrating, wanting to pull out my hair case.

But I've switched 2 others from mice to rats. With my pinstripe, she had no problems and eagerly accepted rats... her problem was f/t (but she still switched).
With my bumblebee, he would take f/t mice right off the bat, and was like my pastave...except once he took his first live rat, that was it. He was a rat eater f/t or live.

Ball pythons are like picky 3 year olds lol