Draco put his hands onto hers which were currently putting pressure on the deep gashes. "A day before the Manor, Voldemort ordered Snape to produce potions that cause werewolves" claws to become poisonous when taken regularly. Originally, he had tried to find a way of turning people into werewolves every time they are bitten by one, not only on a full moon, but it never worked. Or so Snape said. It was more a vision than an actual potion at that point, but I fear that this could be the reason why your spells are partly failing."