I have kind of similar issue.
I am in act 3. Let's say i've done the best choices in the game including those for the companions.
I've started with the green-girl in act 1 because she offered it. I said "why not". It looked like she wants just a fun (fling). Then i dump her (just for sure - in advance before finishing act 1) because i wanted romance the red-K girl. The game hasn't offered me any other romance option since then.
!!! *There might be one major factor during my play through!!! I PLAY CO-OP* (on PC with my wife :)). So, the game might prefer her because she is "the player one" (we both have custom character - started together). Both of us has one companion == less dialog options (companions from the second group are not part of a conversation even though they are "listening". The second group also cannot help with checks/rolls except of buffing in advance). That kind of sucks. =>> I hope they will improve it though there is low chance we will play it for the second time (it is time consuming).
Fun fact: My wife had extra dialog option though she had low approval for the red girl (something about "but i cannot touch you").
Later on when i realized it won't work with the red-K girl i've tried other companions and nothing. So i've decided to become a b***h and having the tentacle experience

(for fun) because the game has rejected me from any other.
Interesting, what it does and how it affects your experience and rollplay.
PS: I am not sure if i am happy with the implementation of the romance in this game. It feels RAW / awkward (sometimes). For example the dialog option with the red-K girl "but i cannot touch you". The gild hadn't told us almost anything about her and there was that option. Imagine a stranger on a street telling you this. Also the "hug" scene felt more or less wrong in the same way. I personally am disgusted with reg girl romance. My wife has successfully romanced Will. It wasn't that bad but my wife doesn't seems to be satisfied with it either. I think we are before a final battle so she hopes there will be something more. So, we will see.
You have to do it before a certain point in Act 1 and rest. Confirmed working. Weird mutual exclusivity though. It would make more sense for Shart and Lae to refuse each other because Shart says so to Wyll.
Yes, I forgot to mention that, you need to rest after it.
Anything before completing the goblin/druid camp slaughter quest worked for me.
I haven't tested if it can be done that way in Act 2 as well, I'll mention it here later if someone not beats me to it.