I think the biggest problem people have switching from live to f/t is how they prepare the food.
We have had the best luck letting the rodent defrost at room temp (86 in our snake rooms) on a thick padding of newspaper to defrost slowly. When fully defrosted rub them dry in a paper towel so it fluffs them dry and creates a little heat from the friction. When we offer f/t done this way we have about a 90% feed success.
If you do the hot water route it can cook the rodent on the inside, and it also makes them have a very strong smell, which a lot of snakes do not like.
Hope this helps.
If not try to leave a f/t rodent in front of its box at night and they will usually take it.