If you want to keep them on f/t, I would NOT offer a live. Most likely, the only thing that will accomplish is to encourage them to hold out for the good stuff.

Some snakes are finicky eaters. It is possible you would not be having any more success on live.

The first thing I would do is double check all of your husbandry. If everything checks out, then I recommend putting some serious thought into whether you want to keep your snakes on f/t, or make a permanent switch to live. If you want to stick with f/t and are consistent about it, I bet they will gradually come around to eating more consistently.