Answer: There are several reasons for lilac not to bloom, but assuming the shrub is growing well, the most common is probably incorrect pruning. Lilac blooms on "old wood" and should only be pruned right after it blooms. Pruning later in the summer will remove the next year's blooming wood and/or flower buds. Other possible causes would be insufficient light (lilac prefers full sun), or overfertilizing (too much nitrogen can cause an excess of leaves and reduced blooming) or less likely but still possible, underfertilizing if your soil is truly terrible.
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