One, are you fertilizing with a product that is heavy with nitrogen? If so, that will grow leaves rather than flowers. A soil test will help determine what type of fertilizer you should buy. It may need more phosphorous – that’s the P in the NPK. Another problem could be pruning. If you are pruning late in the summer, you are cutting off the flower buds for the following year. I try to prune my lilacs by the first of June.
Thanks for writing!