I would say try live with them once and see how they react. A lot of our balls prefer live over f/t. If that doesn't work, try scenting the rat with mice. Sometimes that gets them going too. We've had to do that with a couple of our balls who went off feed for a while and now they're back on rats with no hesitation, some on f/t, some still on live.

Another thing I have success with is warming the rats up pass 90 degrees. If you have extra heat tape, have them dethaw on that. Once they're really warm all the way through, then do movements like a live rat in the tub. That works pretty well also.

Good luck!