I planted irises last fall and only half of them bloomed. Do you know why?

Bearded iris grows from rhizomes, which should be planted just at soil level. If yours were planted too deeply, they may be indicating their distress by not blooming. Wait until July or August to dig and replant them. The soil should be well-draining, with some organic matter worked in. After digging, trim the leaves to about 6 inches and place each rhizome so the top is just below the soil surface, spacing them 1-2 feet apart.