Then it must be due to one of the following reasons
The lilac has been placed in a spot where theres not enough sunlight.For optimal flowering, lilacs require at least 6 hours of direct sunlight.
The lilac has been given too much nitrogen fertiliser (N). This leads to abundant growth but is detrimental to flowering buds.
The lilac is not being pruned correctly. The best time to prune is during or just after flowering. Any of manner of pruning leads to the loss of flower buds.
It can take up to 5 years before a damaged or weakened lilac has developed a good root base. During these years the plant will put all its energy into growing new roots, and not in flower buds.
The lilac is an unknown, un-certified species, so clearly not from the Parfum de Nature brand. Lilacs from Parfum de Nature are species selected for their flowering abundance, colour and strength. They are cultivated in a pot and grafted on specially selected root stock. Root stock that produces hardly any root suckers, but does form a healthy root system.