Hi welsh dragon 45, there are couple of possibilities. First, Lilacs need a sunny position in order to flower well. Second, it could be getting too much nutrients which results in a lot of leafy growth and little flowers. Maybe it's planted in a well-fed lawn? Third, it may have been pruned in the wrong time of year. Lilacs do not need much pruning but when you do it it should be done after flowering.
Another thing with Lilacs is that it's not unusual that - after having flowered beautifully in the year it was planted - it doesn't flower for three to five years. I really hope that this year's flower is a sign fot the future so you can enjoy their wonderful scent.