With Laezel, it's too easy. But I got Minthara into the party, and it's already act three, I'm level 9, and the romance still hasn't started.
Perhaps it's because I went first to the githyanki and then to the Moonrise Tower? The approval of all other companions is very high, and Minthara is missing points simply because she joined later.
You probably missed her romance, then, it starts in Act 1.
To the main point of the thread: romance is not a minigame that you play until sex pops out. Don't assume that you've completed a romance just because you get a sex scene. Larian made it clear that they wanted to portray different attitudes towards sex and relationships with the companions. Lae'zel is very forward, but clearly does not love you. Clearly there is room for emotional development.