Gaynor, I use bait stations. I put them where the chickens can't get near them. The bait stays in the box. But nothing works if there is food available. All my feed is locked up at night including the chicken bowls, and the bait disappears , but there are periods of the bait not being eaten. I figure no one's around to eat it.
Since I've just gotten 3 necropsies and the results have all included some enteritis, I think that's a possibility as well. I had a few with enteritis a few years ago and the rest stayed fine on Corid, and Tylan. The Tylan was given out of my desperation, but it may have saved them from enteritis. I read you can also give them a 5-7 day course of Tylan or ampicillin , etc to clean it out if it's enteritis.
I do admit I'm focused on enteritis, but I think it's still worth a mention. I would also think about getting a necropsy on your next demise to maybe save some of your future chickens.