My first playthrough was the same thing, but I think it’s because I picked her up so late in Act 1. I have no actual data but I think that if you don’t have a certain level of approval with her when you

spoiler about Karlach's side quest

have her heart tuned the first time

you miss out on romancing her for the rest of the game. For my playthrough, I basically picked her up, and started progressing through her quest immediately, and already that the item needed to finish her act 1 storyline; I think that’s what locked me out. Again, I’m just speculating, though.