If you look at a flower bulb, you see small threadlike roots emerging from the bottom of it. Like all plant roots, these take in water and nutrients. The stem and foliage emerge from the top of the bulb. Through photosynthesis, these produce energy-rich sugars that the plant stores in the bulb to power the next year's growth.
Several problems can cause bulbs to die. Bulbs can rot in oversaturated soil or if not kept dry after digging up and storing. They can become overcrowded and suffer from the same stresses from lack of water, nutrients or sunlight that afflict other garden plants.
Take the time to properly prepare the soil in which you plant your bulbs and dig them up and replant them at the first signs of overcrowding. Avoid cutting or mowing green foliage after blooms have died, as the foliage produces the plant's energy stores, and the bulb eventually dies if these stores aren't replenished.