Not enough sun, too much fertilizer, too much moisture and improper pruning are all reasons flowers may not appear on a lilac bush. These problems must be overcome before blooms will reappear.
Preferring full sunlight, lilacs require sun exposure for at least half the day or they fail to bloom. Plants grown in the shade are also more likely to grow fungus, which may affect the blooming of flowers. Remove shading branches from surrounding trees to increase the amount of sun the plant receives. Transplanting the bush to a sunnier location can also solve the problem.
Lilacs do not need to be fertilized. Adding fertilizer to the soil may cause the lilac to concentrate on producing more leaves and branches at the expense of buds. Stopping fertilization often returns the blooms.
Well-drained soil is essential to prevent root rot and general signs of poor health. Healthy plants are more likely to produce flowers. If the soil around the lilac is too moist, develop a drainage system to remove the water, or transplant the lilac to another location.
Cutting a lilac bush after the new buds have set can result in few or no blooms the following year. Lilacs should be pruned as soon as the blooms fade. Do not prune the bush too severely, as this can prevent blooms for several years.