Do you have a name for your iris? Different types need different treatment.

If they are the sort with longish very chunky roots that grow horizontally, they like to have the roots above ground because they need sun on them. Could be those, as they do have large leaves.

Also if the clump bulks up to much they don't flower as well or at all. They then need to be lifted and split into smaller clumps soil fed and the iris replanted.