I'm lucky to have a lot of friends who've been slowly teaching me some gardening stuff, and happily, since I had a lilac in my yard, they taught me about that.

One thing to remember (my friends told me) is that lilacs only bloom on growth since the previous spring, so if you trim your lilac late in the summer or early fall, you're likely to trim all that growth and not get enough new growth to get blooms. My friends taught me to trim lilacs right after the blooming ends, so that they have all summer to put on new growth.

The transplant stress makes a lot of sense, too. Especially if they didn't get a chance to put on new growth, maybe?

Good luck with your lilac!