Make sure it's getting enough sunlight and use a low-nitrogen fertilizer.
Have you soil tested and fertilize if needed.
Lilac bushes that won't bloom could be the result of too much nitrogen. Because of this, you should not fertilize your lilacs. They do not need much in the way of nutrients, and fertilizing can cause a lilac to take up too much nitrogen, which keeps the lilac bush from blooming.
Lilacs need to be 3+ years old before they really get into flowering. They need full sun & well drained soil and not much else! Lilacs do not need to be pruned, but if you prune it back hard in order to rejuvenate the bush, it will take several years to recover and may not bloom during that time.
Yet another reason for a lilac not blooming, or not blooming well, is that it is too old. Lilac bushes bloom best on younger wood and, if your lilac is mostly old wood, the number of blooms will be reduced. You will need to do a rejuvenation pruning on the tree, which will affect the blooming further for 2-3 years, but after that the lilac bush will return to full blooming.
Pruning pruning pruning cut all small growth back and lace out the pruning much like a citrus tree, the flowers are like the fruit and need air between them to circulate.