For years and years I had hundreds, if not thousands, of irises. Each year I would set out boxes and boxes marked "Free Irises" in front of my house because I didn't have room for all of them after I divided them. I even (shudder) threw many away.
The gophers never bothered them as long as they had roses and bulbs to eat. After I replanted all of the roses and bulbs in gopher-proof cages, however, the gophers came for the irises and ate more than half of them.
Even in the areas where the gophers have not ventured, however, my irises grow and bloom, but they don't multiply anymore. This doesn't apply only to the older clumps. The new irises don't multiply either. A few years ago, for example, I acquired lots of new irises and planted them in beds that have never been bothered by gophers for a variety of reasons. Those single irises have bloomed every year since then and still look quite healthy, but they're still single rhizomes. Not one of them has multiplied. This is true of every type of iris I grow: tall bearded, median, dwarf, Siberian, Japanese, Pacific Coast hybrid, and various species irises. What could be causing this?