First, I assume your lilacs are planted in full sun.
You may be over-fertilizing your lilacs. Too much Nitrogen will cause a lot of bright green rampant growth and no flowers. Lilacs don't need much fertilizer and especially not fertilizer heavy in nitrogen.
Your soil may have wandered into the acid range. Lilacs like slightly alkaline soil. The only way to know for sure is with a pH meter but, you can determine which direction your soil is leaning with this DIY test: Mix some dry dirt with some distilled water in two containers to make some runny mud. Add vinegar to one and baking soda to the other. If the vinegar mud fizzles, your soil is alkaline. If the baking soda mud bubbles, its acidic. If you never see any results, it may be neutral or you didn't add enough vinegar and baking soda.
Let us know what happens.
