Have you ever done a soil test, Cathy? Lilacs do not like acidic soil, so that is a possible culprit. Also, if you have used a fertilizer with too much nitrogen, that could also be to blame because nitrogen encourages leaf production at the expense of blooms. Other things that could affect bloom set: the amount of sun the plants are getting and whether they have been pruned.
The place I bought them said to use a 12-12-12photo fertilizer. And trim back old wood?
believe it or not i told mine if they didnt bloom next year they were getting cut down.guess what they bloomed the next year and it make year6 they were in the ground.my husband said well it that works why dont you tell your honeysuckle bush that so i did the next year it bloomed.sounds crazys but it works....BELIEVEIT OR NOT.
Lilacs bloom on old wood so I'm not sure what the place you bought them from meant. Do not prune at all this year and cross your fingers for next, possibly issuing CG's threat.
I was always taught to trim back lilacs just after they bloom because if you trim back later in the season you will cut off future blooms.
Mine have been in the ground about 6 years and just starting to bloom last year and a lot this year. But the bush doesnt seem to be growing like it should....still only about 2 1/2 Feet tall. Was much taller when I transplanted. I don't prune the end of stems just prunt out the old dead or bad stems from the bottom of the plant. So maybe the soil is not right for it. Will try the 12-12-12 Photo fertilizer as mentined above to see if it helps.
http://www.finegardening.com/pruning-lilacs This is a great website, with tons of vital information to help keep your lilacs healthy and producing full vibrant blooms. There are even some diagrams on pruning.