This was reported by a couple of people on the technical problems forum, and I also never had Karlach’s first romance scene trigger on my current playthrough, even though there were many opportunities. So this seems to be a real bug. And my goodness, is it a bad one. You may be waiting a while for a fix, as the last few hotfixes haven’t seemed to do too much.
The folks who reported it on the technical problems forum chose to spend the night with her at the tiefling party, but the scene didn’t trigger even then.
You may want to consider submitting an official bug report to Larian, so that they are aware.
