Change the poison but keep the baiting regime and check it evey week . even if it's not been taken there will still be rats around . Again every few months change even that . As stime goes by expand th bait protected perimeter.
Feeding the grain & how it is stored .
An old fridge laid on it's back that has a fitting door is reasonably rat proof & is useful especially if raised up to crotch height for easy access to the grain . Always clean up any spills , the less easy food also means the less rats.
When you feed the chickens do you have the feed in a feeder that is undercover & suspended by about 14 inches off the floor so that the chickens can only get it by standing and stretching a little?
Feeding them like this is about the best way you can for it helps them to eat any spills off the floor first .
If you have mechanical traps always set them ( with thick bacon rind??? ) no matter how clean you think your place is. place the traps and poison bait along the sides of wall or by the wall at a door way , for rats & mice don't like open spaces. They would rather run along a hedge line , wall or a ditch bank as these afford a degree of safety from predators such as birds of prey .