At least six hours of sun is proper for Lilacs. They also like a sweeter soil than most plants, so a handful of lime every other year or so might help. Make sure they are in a well drained location as well. They do not like wet feet. Deadheading will help, but with a 10 year old plant you shouldn't need to do that. My guess is that they are not in a great location. You know the rule. Right plant, right place.

Thanks, Steve. You may be right. They probably get just 6 hours, maybe a bit more, but it's not all day sun. No wet feet. Soil is fairly acidic so I could put lime down. But neighbor's lilacs--literally 15 feet away--do well and I'm sure she doesn't give them any different treatment. Only difference is that hers are older and taller. They do have to reach for the sun a bit because of shorter shrubs in the way, hence the spindliness, but I still don't know why it would be exactly every other year that they blossom--is that a stress response?

I've never heard of it in lilacs, but some fruit trees use up their energy or nutrients the years they flower and set seed, and thus can't produce buds that Fall to bloom next year.