The bigger you let your clump of iris get, the fewer flowers you will have. I try to divide mine every 3-5 years, depending on the growth habit of the plant.

Also, I have found an iris in not enough sun will grow and spread happily but not bloom much at all.

I give my irises a bit of bonemeal every couple of years and make sure they don't get really dry for too long.

I agree with the dividing, all of mine need to be done this fall. I also believe if they are planted too deep they won't bloom. You need to see the rhizome. C