Can't Romance Sera

I've had similar troubles way too often: I can take every flirt opportunity, do all her War Table operations and Inner Circle quests, and max out her approval, and even then it's more likely than not she'll never start a romance with my Inquisitor — even when I'm playing a Qunari. It's frustrating as #@*!, to be honest, especially since I can't see any difference in what I'm doing when she does.