Far as I can tell, the game *really* wants you to have a happily ever after ending in romantic pursuits, so it's pretty hard to completely fail. As long as that person's Attitude is high enough, you should be able to progress it, maybe when you reach the right level, or when you get to the next Act.
But then I might be wrong
Personally I stayed away from any "romantic" exchanges. Call me old-fashioned or whatever, but I really prefer the more subtle, elegant, and gradual romances in games like Baldur's Gate or KOTOR or Dragon Age. Romance in DOS2 is... ehhhhh.. not for me, I guess. Or maybe I'm just old.
