Wait until mid summer, it'll never work until it warms up. But hopefully by July or August their appetites will be picking up again.
After that, you just have to skip weeks. You want this f/t mouse? No? Then I guess you're not hungry enough. Maybe next week. Still no? I guess you're still not hungry enough...
I actually would offer them live, but only every other week. And only hoppers or small adults. And I would offer them a fat f/t as seconds--sometimes they would take the f/t as a second helping, but not as a first.
My 2007 het lavender male took SIX MONTHS to switch and he relapsed more than once. But he's been taking large f/t mice steadily for almost a year now, and he's beginning to put on real weight. (He's still half the size of his girlfriend, who's also a 2007 and -only- took 2 months to switch to f/t.)