This can be caused by a number of factors and usually a combination of more than one. For example, planting in too much shade. Overfeeding with animal manure or blood and bone, or the reverse, no feeding of anything. Planting too deep can be a problem. Cover only with 1 to 3 cms of soil. Planting so high that the rhizomes are exposed to extreme sun and wind blowing them around . Don’t mulch the irises, mulching with lawn clippings or other organic matter acts as shade and can cause them to rot. A soil mulch is sufficient. Over or under watering can also cause problems. They need replanting every 3 to 4 years or overcrowded starved Iris will not flower.