I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
Your lilac seems to be stressed most likely from either a lack of sunlight or possibly from poorly draining soil. If these are not the case then see how the fertilizer boost you used works to get vigor going again. Water to keep the soil moist but not soggy. Sometimes after a mild winter lilac buds don't emerge properly and there is little you can do. However that variety requires very little chilling compared to most.
One last thought. Don't prune it after mid summer or you could be cutting away bloom buds for next spring's bloom season.
Thanks for the question. Please stop in again soon!
Q&A Library Searching Tips