Sleeping elixir glitch?

[Naelle]

I have the Supernatural expansion pack and I've tried both normal and potent sleeping elixirs on two vampires and a normal witch. The vampires got tired and fell asleep while standing a few times but the potion stopped having an effect once they weren't tired anymore. But my witch kept on fainting every once in a while even though she wasn't tired anymore. Does anyone know why the vampires weren't fainting?