Some possibilities to consider and rule out... Ensure that the soil is not bone dry by testing the soil often and watering if necessary. Lilacs may start going dormant soon in some locations. Determine if the shrub is not getting more than 6 hours of direct sun. Since lilacs bloom on dormant flower buds produced in late summer/fall, determine if the plant is being pruned after it developed the flower buds. Ensure the soil is not kept soggy or the lilac may abort the flower buds nor develop any/many flower buds. Do not use high nitrogen fertilizer; the last application of a slow-release fertilizer should be made about 3 weeks prior to late October/early November (your average date of first frost). Late frosts may damage the buds so they do not open. One-time only, consider giving new plantings a "passing grade" in year one in case they are too busy developing the root system for some reason. Older plants may be root bound. Very young/small plants may require a lot of phosphorus to bloom.
