Answer: Lilacs may bloom less if they are in need of more sunlight, if they have been pruned at the wrong time of year, or if they have not been pruned regularly over the years.
Routine pruning would be to remove the faded flowers and to remove any suckers smaller than a pencil each spring. After bloom, also remove some of the oldest stems by cutting them at the base, at the ground. This causes the plant to constantly renew itself with new growth coming up from the roots and stay vigorous. It also helps it stay shorter and bushier.
Some lilacs bloom heavier one year and lighter the next on an alternating schedule. You may find that yours is one of these.
Six hours of full sun that include the hour of noon would probably be enough light for the lilac although they do best in full sun all day. Since you mentioned a tree, there is a possibility that the lilac is within the root zone of the tree, which can reach at least as wide as the branches of the tree. If this is the case, you might want to try to transplant it.
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