The main reason for no bloom is not enough sun. Six hours of sun is usually required, although you'll get fewer blooms sometimes with less sun.

Other issues could be wet feet. Lilacs really don't tolerate wet feet very well. Sometimes very acid soil (pH 5 or lower) will cause less blooms. Lilacs like neutral soil so a handful of lime might be in order if you know your soil's pH. Sometimes young plants won't bloom unless spent blooms are removed, causing an every other year pattern. Sometimes homeowners "prune" the blooms off of a plant through improper pruning.

How is the growth of the plants this year? Did you get any dieback over the winter? Any dead limbs? I had two plants not bloom due to that problem this year. So I'm cutting back some of the older branches.

Steve

They are young bushes under 5 yrs old. I did not prune them at all. They have bloomed in the past,but I did not remove the spent blooms.
I did not have any die back and they leafed out well and look good. Do not know the PH of the soil..I will have to look in to that...I doubt it is wet feet..the area is not in a damp location.

When very young....or very old, shrubs can go into biennial bearing mode - after a really great flowering/fruiting year, they have to rest the following season...and recoup.

This sort of behavior that campanula describes happens with my Rhododendron bushes, for what ever reason that may be. This year it decided to bloom; I'm away from home currently but the flowers were so profuse and impressive that my dad called me just to gloat.

As for lilacs... I've come to find that they really do enjoy a nice pruning immediately after they've bloomed. There was a disgusting, uncared-for lilac stand in between me and my neighbor's yards which was littered with dead branches and sucker-like growth. It took two years to trim to bushes back to a pleasing shape and height but they've enjoyed the haircuts. What impressive flowers this year!