I've had this problem and to be very honest some snake won't switch no matter how much you try.
1. have you tried to feed f/t and failed?
If so place the snake with the pinkie in a small sealed container over night or for a few hours and see what happens, I've had better success with over night.
2. How old are your snakes?
It isn't impossible to get older snakes to take f/t but it is more difficult.
3. Defrost the food item and using a pair of forceps move the prey item around so that the snake thinks it's live.
4. Try using pre-killed 1st so the transition isn't as drastic.
5. Offer a live item and a f/t immediately after.
6. Start a colony: breed your own prey items.
7. Lizard scented dead pinkies may work as well.
#1 and #4 has work for me with some snakes not all. i have a small colony of mice that supply food for 2 hypo Hondurans(pre-killed [i'm attempting a gradual switch]), and i give others a "live treat" every now-n-again
Hope this helps!
~ZF
