What would work better than fertilizer to make those iris bloom would be to divide them up and replant the more vigorous side shoots and get rid of the older middle section of the iris roots.  Do not plant the new divisions too deep but just below the surface of the soil. The best time to do this is right after they bloom but you can also divide iris in early spring. Use a knife to cut at the knuckle or joints. In May, or at the time you do the replanting  you can add some slow release plant food such as Osmocote around the base of the plants. You may not notice more blooms this year but I am betting the following year you will enjoy bountiful blooms. Keep growing, Marianne Binetti