Not adequately established
Some iris cultivars need a year or more to fully establish in their new locations before blooming. If you relocate them frequently, they may never become well enough established to bloom. Plant irises far enough apart to allow for several years' growth before requiring division.

Inadequate sun
Bearded irises need at least 6 hours of direct sun a day to bloom well.

Inappropriate watering
Bearded irises might not bloom well if they experience periods of extended drought, though the plants themselves are quite drought-tolerant. Conversely, bearded irises that are over-watered are often susceptible to bacterial soft rot and fungal leaf spot infections. If you provide supplemental water, water deeply no more than once a week. Soaker hoses are preferable to overhead watering to avoid spreading leaf diseases from plant to plant.

Planted too deeply
Bearded iris rhizomes should be planted so the tops of the rhizomes are at or slightly below the soil surface. If planted too deeply, bearded irises will grow leaves but may not flower. Be careful, also, not to allow mulch to cover the rhizomes. Make sure any mulch is pushed away from the rhizomes.

Overcrowding
Overcrowded clumps often quit blooming until they are divided, OR irises closely planted with other plants may not bloom well (or at all) if they are struggling to compete for sunlight, water, and soil nutrients.

Weeds
There are certain weeds and grasses that are so aggressive they can inhibit the performance or even survival of plants they invade (Canada thistle is one of them). Keep the weeds and grasses away from your Irises.

Ill health
Irises that are diseased or under insect attack may not be able to bloom until the problem is eliminated.

(Mediate Editor's note: Through the eastern US, iris borers are a major problem as they can not only move down through the fan and the rhizome to give rot a foothold, but often move up through the bloom stalk and destroy it or the buds. The usual advice is to spray with Cygon, but reports in the AIS Bulletin indicate good success for some individuals with applying other systemic sprays, burning old foliage during winter, spraying soap solution on fans, and scratching granular systemics (Merit) into the soil. And some swear by pinching out baby borers early!)

Late freezes
If a late freeze occurs when flower stalk development has already started, the stalk may abort. If a late freeze occurs when flower stalk development has already started, the stalk may abort, (even if the stalk is not showing yet. Killing freezes that are severe enough to damage iris foliage within 6 to 8 weeks prior to normal bloom can abort stalks.)

Immature rhizome
Rhizomes will not bloom until they are mature. If you have planted smaller rhizomes, you probably need only wait for them to grow a bit before they will bloom.

Irregular bloomer
All irises are not created equal. While some irises may bloom very regularly in your garden once established, others may never do any better than blooming once every several years ... or perhaps never blooming at all. The same cultivars that bloom beautifully and reliably for a neighbor down the road or a friend across the country may do nothing more than sulk in your own garden. The only way to discover which irises will perform best for you is to keep trying different cultivars, growing them properly, and replacing those that don't meet expectations within 2-3 years after planting.