The GARDENA gardening expert
Dear Mr. E., without seeing the problem for myself, I suspect that you may well have pruned it too severely, and thus cut out the budding shoots of the lilac. For lilacs which have reached their flowering phase, it is basically physiologically impossible for them not to bloom every year.
My advice would be as follows: Adapt your pruning over the next years to the new shoots and the will of your plant to flower. Observe here the growth and flowering characteristics of the lilac: How does it regenerate? Which new shoots flower, and which does the plant use for structural purposes? In this way, the lilac will teach you through its growth and your observations which branches should be harvested in the interests of retaining its flowers, and which should not be harvested.
Another thing, dear Mr. E.: As you have seen in your garden, lilac takes root very close to the surface, with a dense network of roots. If these roots penetrate too deep into the soil – either because soil is added during the course of garden work, or through the excessive addition of bark mulch – the roots are no longer ventilated sufficiently. Physiological problems arise, which lead to a lack of flowers and eventually to the death of the plant. In one case known to me, the addition of approximately 20 cm of earth onto the roots led to a plant dying off completely within three years.