The map on this page gives you a visual of this plants' range, you are at the warmest edge of it in zone 7. The especially warm conditions last winter (chill hours) were probably just not enough. That's why they aren't grow down here, they need a certain amount of chilling (resting.)

They were doing so well prior to the last few winters. Is there any way to compensate for this?

Lilacs NEED a cold winter in order to produce flowers. There is no way, in your zone, that you can 'compensate' for warm winters. You've discovered why few people grow them in milder climates.

The next time you get a real winter, you should have good flowers the following spring. Oh how I miss lilacs!

I agree with some of the comments above, that the warm weather this winter may have been the culprit. The lilacs here have not made their very best show. Many of the clusters are smaller than they have been, some are cloven for ways in a starfish like way, and others are just miniature puffs of flowers. All of these are beautiful in their way, but I can see there is not as much bloom as in colder years. Such is life. The other unusual thing is that lilacs are blooming here in New Jersey, when usually we enjoy them two our three weeks later in May. Here is something telling. Winter, although there was some cold weather simply was not very long. You are familiar I am sure with the concept of chill hours. Lilacs need cold to bloom, and they also need it over a prononged period, say three or four months. Spring began with Groundhog's day this year for us, which was perhaps a lovely trade off.

Lilacs grow perfectly well in Zone 7, but keep in mind that the middle zones, 6 and 7 are two entirely different things North and South. Here at the Jersey Shore, we have rare winter mornings in the upper teens. It almost never gets to the 5-10 degree range, but that is possible here. The difference is in the South, for example Alabama, winter does not involve feet of snow, and ice. This will flatten an otherwise hardy gardenia or camelia japnoica that is either in a container, or not in a protected spot.
Lilacs don't mind though!

I wonder if gibberellic acid could do anything, or if it'd even be an affordable option.

No to gibberellic acid. As has been said, you cannot compensate for the lack of suitable temperatures.