depending on how old they are, some tough love might be in order. If they are older and can handle a few weeks-months with no food then simply stop giving them live. If they are hungry they will eat.

I have also heard of people pre-scenting with a live rodent and offering the dead one. Some people even go as far as to keep the rodent in a closed transparent container inside the snakes habitat. That way it sees the prey item and is eager to eat. Then you wave the f/t at them.

It has been my experience that it is not that hard to switch from live to f/t and back again. I have switched a few times and no one has had a problem taking either.