Whats wrong with my Lilac, it was beautiful last year. The leaves are there at the top to come out but only the bottom ones are out in full and I have 3 blooms at the bottom. Its against a west facing wall
There have a been a few threads on here this spring reporting poor flowering or odd growth on lilacs - for example here: http://www.gardenersworld.com/forum/problem-solving/lilac/996378.html
Maybe the weather pattern this year has just not suited them.
It would be worth checking to see if it's grafted. See if the strong leafy growth is from below the graft and the poor bits are above the graft.
It's against a west facing wall, but the part that looks poorly is taller than the wall and therefore exposed to the freezing north-easterly winds we in the East of England have endured recently ... maybe that's what's done the damage? Whereabouts are you, geographically speaking?