I highly doubt that liming the soil underneath the lilacs is going to solve
your problem. Lilacs supposedly prefer neutral soil, but they do fine just
about any condition. The plants are still small, as you noted, and might not be
old enough to produce flowers yet. There is also the possiblitiy that the
flower buds died over the winter from the cold. This dosent happen often but
it is possible. If you have been pruning them, there is also the chance that
you cut the flower buds off both years. I would get a soil test done before
you use any sort of pH altering material.
Toad