Iris buds curl up & wilt.

Hi everyone. I have a selection of different irises in my garden which I planted 4-5 years ago. They are all in the same place, a long thin bed, and have multiplied, producing lots of large, colourful blooms. Last year and now this year again some of the buds are failing to fully open and are either shrivelling up or turning slimy & deformed.
Does anyone have an idea about what might be causing this? It applies to all the plants, whether I've brought them indoors as cut flowers or whether they are still in the ground. There don't appear to be any obvious pests on them. Could it be an overcrowding issue? I am considering splitting them. When would be the best time to do this?
Many thanks for any help. lindyco