Could be the prey you're offering is too small for him to feel like he needs to bother killing it. If he can simply "suck them down" then they're probably too small.
As for the f/t issue....how long will you let him go hungry? Do you offer live right after he's refused a f/t? If I were trying to get a snake to switch over, I would stop offering live altogether. Wait 2-3 weeks...then offer a nicely warmed thawed critter. If they refuse, I'd wait another two weeks....let them get gooood and hungry. I would not give in and offer live unless it got so bad that the snake started visibly losing weight. Especially an animal like a burm, that will typically eat anything remotely edible.It may take a few tries to get him to recognize the different smell of frozen/thawed prey...but once he does, I'm sure he'll take to it just fine.