Hello, in this time, have you ever pruned the liac, lilacs all ways do well to severe pruning and soon send new shoots from the pruned wood, and yes they will always put on lots of nice new foliage, but this can affect flowering for a few years, are the lilacs being starved of sunlight from other trees etc, lilacs are better at flowering in the sun rather than shade, hard pruned one, four years back, no flowers in this period, but in flower this year and this lilac is in an open sunny situation, julien.