I'm pretty sure that's a bug. I've noticed the same thing, and the method I've found for resolving it is to right-click her. If she gives you the "let's talk" dialogue, then she actually wants to talk. If she gives you anything else, you're experiencing the bug.

You have to get several thousand points of affection with her before she truly begins to interact with your character. At around 3000, you start gaining affection with her with pretty much every conversation interaction.