Iris going downhill

[Nicos]

I've tried growing bearded iris all over my gardens both front and back, and only 2places seem to be successful.
Both are positioned where the sun can bake dry the rhizome in summer and they are not sitting in cold wet soil in the winter (makes them shrivel and rot)
No idea if that is helpful, but certainly worth bearing in mind.

[shushkin!]

I grow quite a few bearded iris. I find them pretty easy as long as you provide the right conditions.
You seem to be doing the right stuff. It does sound like something is having a go at it. Maybe drench it with an insecticide. I do give mine a handful of fish, blood and bonehead every year and they seem happy enough. My south facing back garden is perfect for them.