I also suggest maybe trying live. One of my boas wouldn't eat F/T for a while and he's too small that I didn't want to risk him losing weight, so I gave him a live hopper mouse and he struck immediately. I think next time he feeds I'll do a prekilled mouse and then F/T and hopefully he takes it. Perhaps you could try the same?
Or I've heard that dunking the rat in chicken broth can help because they like the scent