I have a lilac bush that's at least 18 years old. Last summer I cut it back for the second time in 5 years to about 5'. This year the growth has been more prolific than ever and it seems to be spreading at an ever-increasing rate. However, it hasn't flowered since that first pruning 5 years ago. What am I doing wrong? Why isn't it flowering?
Let's assume the shrub is in full sun, 6+ hours of direct sunlight. Then I would look to your pruning technique. Most spring flowering shrubs do better is 1/3 of the oldest, largest stems are removed each year after blooming. Cutting back to 5 feet does not give the same results. Old stems and branches are the least productive of flowers and leaves. Removing some of these big ones allows more sunlight to reach the interior of the plant, another good thing. Also, thin the new stems coming from the base and from the pruned-back stems. If not done, stems shade each other out and become so crowded that foliar disease risk increases, including powdery mildew. You want the sun to reach as many leaves as possible. Do not prune after July 4th since you will remove developing flower buds. I hope this helps.
There can be several reasons for a flower failure!
This article will help you assess the plant.