The only yellow lilac is a cultivar called "Primrose." It should bloom fine. Lilacs should have 6 hours of sun, so you are probably right that it doesn't get enough. You can also try putting lime down now for next year's blooms. Lilacs prefer a neutral soil and here in NE we have acid soil for the most part. The other issue might be wet feet. Lilacs come from mountainous areas and are drought tolerant once established but they will not tolerate poor drainage.

Steve

Don't know much about yellow lilacs but for lilacs in general to enhance flowering apply wood ash at the drip line in the Fall. Less than a cup should do.