Are you defrosting then heating them back up again - a lot of snakes (at least to begin with) will not take f/t if they are wet and/or below normal body temperature - make sure they are heated up to about 98* and then wiggle them using hemostats. Depending on their age they may take to it fairly easily - if they are older sometimes it takes work.
Usually when I get one that is stubborn to switch - I'll start by feeding them live off of the hemostats - when they take that readily - I switch to prekilled when they are on those readily I move on to f/t warmed up.
Some will not, no matter what you do, switch over to f/t - in which case you're stuck with a live feeder.
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Tosha 
"Nihil facimus sed id bene facimus"
7.33.0 Ball Python (Harry and Fluffy and gang)
1.0.0 Angolan Python (Anakin Skywalker)
0.0.1 Green Tree Python (Verdi - yeah I know but my kids love the book)
0.2.0 Feline (Pippen and Pandora)
0.0.1 Desert Tortoise (Pope John Paul aka JP )
2.2.1 Fish (1,2,3,4)
0.0.2 frogs rescued from pool skimmer