You have been remarkably patient with your lilac tree! After 14 years it should be full flowering every year. 

Light and growing conditions for lilacs

If your lilac tree is growing in heavy shade, this can significantly reduce the trees ability to produce flowers. Lilacs prefere plenty of light, so removing surrounding vegetation may help allow more sunlight to each the plant.  At fourteen years old, the tree would be too difficult to transplant to another site, so mimproving the light around it is usually the better option.

Keep the ground around the base of the tree weed free and add a layer of Daltons Premium Mulch and Feed to help maintain soil conditions. 

Fertiliser and growth balance

Do not to over fertilise your lilac with high nitrogen fertilisers during the growing season. High nitrogen encourages leafy growth, which can come at the expense of flowering.

Unfortunately, even severe pruning is unlikely to trigger flowering in a plant that has not performed well for many years.

Replacing a poor performer

Ultimately you might be better to remove the poorly performing lilac and replace it with a new one. When choosing a replacement, visit a reputable garden centre or nursery and ask for a free-flowering cultivar of the lilac species.