Why can't I feed on Heather Poe?

[Gopher, King of the Bat Scouts]

I can do it the 'normal way' but not through dialogue, I'm at Skyline apartments.

[Vepar]

I only had it show up occasionally in dialogue, not sure if it's related to anything, but try to spend your blood before you talk to her, maybe it'll show up then.
It either shows up as part of the normal interaction chain with her, and isn't repeatable, or shows up when you're hungry and low on blood.
Not sure.

[Wesp5]

You can't always feed on her, you want to keep your ghoul healthy ;).