The flowers of spring and early summer flowering shrubs bloomed for a long period of time due to the cool temperatures. It was a very good year for most lilacs! If you have an underperforming lilac it may be due to one or more of several factors.
Lilacs planted near trees may be getting shaded out as the trees mature. Removing overhanging branches of nearby trees might be enough to get those shrubs back into the sunlight. Lilacs also weaken as stems get old and woody. Rejuvenation pruning helps here, cutting a third of the shrub to the ground on successive years to promote new growth.
Lilacs pruned at the wrong time will underperform as well. It is best to prune and shape lilacs right after bloom time. Cutting stems of blooming flowers work as well and has the added benefit of having bunches of the sweet-scented flowers to enjoy indoors. Another factor is over-fertilization. Lilacs require very little or no fertilization and overfeeding results in the overproduction of green foliage and a very little flower.