Bulbs are producers of flowers and survive based on a simple principle: save food for flowering, flower, then save food for next season. They put survival of the species first and produce flowers only if conditions are right. Fortunately, most of the time conditions are right, and the flowers bloom right on schedule every year. When they don't bloom, usually a few simple techniques will set things right.
Let Them Grow
Flowering bulbs have to replenish themselves after flowering. They spend all their energy putting up leaves and flower stalks and producing flowers, and finally after all that, they replenish their food stores for next season. Don't cut back foliage until it dies back naturally. This can be a week or two or up to three months. Let the plant take care of itself, and it will flower.
Most bulb plants need some room to grow, and they also produce new bulbs called offsets from their main bulbs. If too many offsets are present, the plant can't grow right and may not flower. Other bulbs and tubers, or rhizomes, such as irises, require regular division to continue growing. Divide these bulbs and tubers on a regular schedule, usually every two to three years. Bulbs with offsets are simple to manage. Dig them up after the foliage dies back, pick off the offsets and replant the parent bulbs. Plant the small bulbs in a separate bed to grow into mature, flowering bulbs.
Fertilization can cause problems in two ways. If you over-fertilize, the plants may not flower and simply produce large quantities of foliage. If you under-fertilize, the plants can't produce enough energy to flower. Most bulbs don't need heavy fertilization. Feed them once per year, and they are happy flower producers. Never feed them, and eventually they will die off. Feed them too often, and they get fat and happy and just grow leaves.
If your bulbs that don't flower are in a heavily shaded area, they don't get enough sunlight to produce food to grow. Dig the bulbs up and move them to a sunny location. Remember that sunlight is seasonal commodity. The bare trees of early spring might be perfect for growing tulips, but your dahlias won't grow there in the summer shade. Put your bulbs where they get enough sunlight every day during their growing and flowering season.
Certain spring flowering bulbs such as tulips, daffodils, crocus and muscari need a cold period to flower. These plants are adapted to cold conditions and flower on a predetermined schedule, right after the cold is gone. That way they don't compete for sunlight and insect pollinators. If you live where it is warm year-round and your spring bulbs won't flower, dig them up and keep them in moist peat moss. Eight to 12 weeks before planting, move them to a refrigerator until you plant.