Is the top of the corm exposed? If it is covered with soil that is probably the problem.

Agree with zen...
but, it could be a few things.
First: Check to make sure the top half of the rhizome is exposed... no dirt, no mulch
Second: If they haven't been divided in a while, they lose vigor. Cut out the old rhizomes, no longer leafing out.
Third: Check for Iris borer

Thanks for your help. I'll pass the information along to my friend.