So, bought Dawnguard off of Steam today and made it far enough along in the vampire questline to where I'm told by my quest journal to use vampiric seduction on this moth priest guy who was being held behind some magic barrier. Unfortunately, every time I try to use vampiric seduction, it'll hit the moth priest and say that he "resisted the spell".

Every friggin' time.

Why would it tell me to use a spell/power that's too weak to actually work?

Is this a bug or is there a work around for this?