I had a beautiful Iris plant that I thought needed to be divided. A couple years ago I dug up the plant and divided it and replanted. Now there is minimal flowering and what is even crazier is that the plant is growing in a large circle with a totally empty center. I'd say the circle has a diameter of 18-24 inches and the skinny leaves seem to be growing on the circumference.
Certified GKH Gardening Expert
I have listed a couple of great articles.
It's possible they are still crowded.
Planting to deep can cause issues with rotting bulbs.
To shallow may decrease blooming.