Lilacs fail to bloom when they receive too little sun or water, excess fertilizer or too much pruning, according to Gardens Alive! Climate also affects lilacs, which need cold weather to thrive.
Lilacs need a minimum of 6 hours of sunlight daily to bloom, while even more time in the sun is preferable. The need for sunlight also creates a strong need for water because buds have the potential to dry before they bloom. Commercial fertilizers often cause lilacs to produce more foliage but fewer blooms, and a switch to compost may produce more flowers. Lilacs can be pruned as soon as flowers droop, but pruning later in the season risks cutting off new growth.