We're guessing that the most likely cause in your case for no blossoms would be that the bush is still too young to flower. Some lilacs may take up to 5 or 7 years to start blooming.
Although you have followed the directions which came with the bush, it might be good to double-check that the plant is still getting at least 6 hours of sunlight (has other growing vegetation started to shade your bush?), and that the soil is slightly acidic to slightly alkaline (pH 6.0 to 7.5). A fertilizer high in nitrogen promotes leaf growth over blossoms; if you need to fertilize, choose one whose N-P-K ratios are about even, and only apply a small amount; lilacs don't do as well in rich soil. Also, make sure that the plant is getting enough water (about an inch per week).