Granted, I'm newer to this with bp's but since he's taken pre-killed before without an issue and yet is refusing f/t, I would wonder if this is a prey temperature issue. You don't want to get the rat/mouse too warm but at the same time, not warmed through can cause them to refuse as well. I've switched all 4 of my bp's straight to f/t without an issue. Every single refusal I saw was due to a rat not being warm enough or being too warm. Once I warmed them up or let them cool down properly, the bp would take it right away. The prey should feel slightly warm but not hot. Personally, I use my dragon's heat lamp to get them to the right temp and just go by feel. If it feels like a live rat in my hand, it's good.