I have only had one snake out of all mine that refuses to switch to F/T. A great way to switch them is to ask a pet store for some soiled rodent bedding. You can thaw it out in the fridge over night with some of the bedding. It will make the rodent smell more appealing. Make sure the head and body is nice a warm to give the snake something to aim for.
Using tongs to make the rodent do the zombie dance simulates a live rodent as well. If you don't have reptile feeding tongs you can pick up a cheap pair of grilling tongs to use for a dollar or two. It doesn't work as smoothly as specific feeding tongs, but it gets the job done.
Patience is probably the biggest key to switching over reluctant snakes. Good luck!