Did you prune them last year??? Most of ours were wacked with a hedge trimmer before we got the house. This is our third spring and finally almost all of them bloomed, but still a couple have not. Here's a pertinent quote from Wikipedia, along with some other interesting stuff:
"Lilacs flower on old wood, and produce more flowers if unpruned. If pruned, the plant responds by producing fast-growing young vegetative growth with no flowers, in an attempt to restore the removed branches; a pruned lilac often produces few or no flowers for one to five or more years, before the new growth matures sufficiently to start flowering. Unpruned lilacs flower reliably every year. Despite this, a common fallacy holds that lilacs should be pruned regularly. If pruning is required, it should be done right after flowering is finished, before next year's flower buds are formed. Lilacs generally grow better in slightly alkaline soil.
Lilac bushes can be prone to powdery mildew disease, which is caused by poor air circulation.
The wood of lilac is close-grained, diffuse-porous, extremely hard and one of the densest in Europe. The sapwood is typically cream-coloured and the heartwood has various shades of brown and purple. Lilac wood has traditionally been used for engraving, musical instruments, knife handles etc. When drying, the wood has a tendency to be encurved as a twisted material, and to split into narrow sticks. The wood of Common Lilac is even harder than for example that of Syringa josikaea."
I'm so glad you posted this and I checked Wiki as I would like to prune some of them for shape so now should be a safe time to do so.
If you haven't pruned, they could just be resting. Certainly we got enough cold and snow this winter.
