Most of the time, the elixir works just fine - some times, they'll get stuck in this limbo between normal Sim and the occult state, and they may have characteristics or motives of both, etc. It's possible to work around, and it may be as simple as hitting them with another potent cure, or going back to the old file and trying again. It's a random glitch, that's all. (Oh, for an in-story example, I think gen. 6's spouse in Tilia's Classics was a glitches vampire, but she managed to figure it out.)
Here's hoping that whoever you choose to use cooperates with you,