I always wondered about that, cos Ava had full control over them, and Sam could have gained that control, but then YED went and he never got the visions again or anything. It was like they died with him, which is weird, cos wasn't it the blood he drank as a baby that gave them to him? But Sam's blood!powers aren't the same as the other powers the kids had, so they must be brought on differently, or Sam's just using them differently... Oh I don't know.