Another suggestion would be to switch more slowly: i.e. from live to pre-killed, THEN to FT. Every snake I own takes either FT or PK, and granted some will just NOT switch to FT...but it beats having to supervise 14 live feedings. And, if I offer prey in the right order nothing gets wasted even if one or two refuse.

The key to switching prey is to be patient and consistent. Keep them on the same schedule, even when they refuse the new item, and keep offering the new item. It just stresses a snake to no end when you try offering another prey item only days after the previous, and if you cave in and go back to live they'll learn that all they have to do is wait and they'll get what they want.

Personally, I would try offering a pre-killed item first since it's typically still warm and possibly twitching when you offer it. That warmth and movement still attract their instincts, but it also gets them used to eating something not fully living as it were. From there, it's a lot easier to go to FT.
Also, remember that some snakes just never switch and never will. We just have to go with what we're given.