Doesn't explain why Barbara lost the power, though. She hadn't needed or even had the plant itself around for years at that point; Circe gave her the ability to access that power with just a drop of blood, completely regardless of any kind of sacrifice, ceremony, ritual, or anything. The only thing that can explain her theft of Tisiphone is if she genuinely lost the power to transform, and the only way she can have lost that power is if Sebastian found some way to genuinely steal it, because all she needs is blood, and blood has never been in short supply.