Have you tried offering the rats at night, in the dark? If the snake is obviously interested (lots of tongue flickering, staring at the feeder) but not striking, I'd turn off the lights and leave them alone for a few minutes. Usually 5 isn't enough for some so you might just be pulling the feeder before they get themselves worked up enough to take it. 20-30 minutes is pretty much the limit for how long I'd leave a live feeder in for, though. After that, it's obvious the snake isn't hungry.