Too much nitrogen can cause vegetative growth at the expense of blooms. Japanese tree lilacs can be inconsistent in their bloom production from year to year. Some years they may bloom heavily and other years not at all. If the roots of the tree are being disturbed by planting annuals this could be a problem. Lilacs need full sun and well-drained soil to perform well.
Here is an article on the most common reasons lilacs fail to bloom:
Lilac Failure
Two other factors may be influencing the blooms. If you are pruning for size, make sure you prune immediately after the time the tree would normally bloom so you don't cut off next year's developing buds. Additionally, a soil test will indicate if the pH is correct. Lilacs prefer to be in a range of 5 to 7.5.
