Are you moving the rat boxes regularly? Rotating bait types as they stop being interested in one type? Setting snap traps or glue traps to count numbers? Have you considered using tracking powder?
@ChicKat
We have both rats and mice. They were both showing up indoors within 24 hours of eachother. So it's probably more of a sometimes it happens sometimes it doesn't.
Here's some advice I'm copypastaing from a post I made in another thread. This is largely from the professionals dealing with my rat problem.
Secure your feed. Don't leave food out or in the coop. Only feed as much as the chickens need each day, pick up excess at night and keep it all in metal...