Experts and BHG readers answer.
No Blooms on Lilacs
Lilacs generally don't bloom for one of two reasons. They are either getting too much shade. They need at least 8 hours of direct sun a day. Or, they are being pruned at the incorrect time. They should only be pruned right after they flower, in late spring. Also, don't feed them too much. Fertilizer will promote lots of green growth, but few flowers."