Experts and BHG readers answer.
Lilacs can be very stubborn about blooming after they've been transplanted. In fact, I had three lilacs that took almost 6 years to bloom after I planted them. All you can do is be sure they are getting 6 to 8 hours of direct sun a day and that you aren't pruning them at the wrong time (only prune in late spring after lilac flowers fade) and don't overfeed (too much nitrogen will cause the plants to grow foliage and no flowers). Otherwise, you just have to be patient."