Do you prune it? I can't remember if it blooms on old or new wood...

You could try root pruning next spring to shock it into blooming.

It is possible, if the tree was grown from seed, it is not old enough to bloom yet. I don't know how long the juvenility period is for a JTL.

Japanese Treelilacs bloom on old wood. The pruning suggestion (don't prune) would be my first recommendation.

Is it in the lawn that you fertilize with nitrogen diligently? Do you fertilize it diligently? Another don't do, IMO. A couple fertilizings of the lawn shouldn't hurt, but no need to feed the tree. If it is still growing fast (1.5-2ft/yr), it certainly doesn't need any help growing. More fertilizer helps keep it in the vegetative mode, not the flowering mode. If you've been watering, it's way time to stop that too. A little drought now and then will help it flower too.

It's too late to do anything drastic, flower buds for next year have already formed, if they were going to. Save the root pruning for spring 2009, about the time that regular lilacs finish blooming. If it works, you will get blooms in 2010.