Hi, thanks for the question. 

Sparse blooms could be due to a few different things. These lilacs take a few years before they bloom prolifically.  They need about six hours of sunlight to be able to produce a good number of blooms. They do need some cold protection in winter from very low temperatures than could damage buds.   Too much nitrogen (like lawn fertilize) will make the plant foliage flourish with fewer flowers.  Pruning should happen Immediately after bloom.  Remember that "boomerang" flowers on both old wood and new wood.  Pruning at the wrong time could remove flower buds.  

I hope this helps.