What determines if a sim will say yes to drinking from them?

I've had my sim had at least two different humans give her a permission to drink from them, and their relationship was only about half way green. But that was in the beginning. Now the relationship is even higher, and neither of them let's my vampire drink from them anymore. I have no idea what's going on.

I was thinking of giving her the Guilty Drinker weakness, but I can't do it now. She won't survive on plasma packs and plasma fruit only, and I don't want her to feel guilty all the time.