Sometimes advie powers are completely useless. Like Debra with the wings over there. She can't fly. Her wings don't work. They can flap but they don't actually lift her off the ground or anything. This happens more often than you'd expect. Like a mind reader who can only pick when someone's thinking about song lyrics. Or some non-functional physical mutation. And it's rather arbitrary. It's the nature of advie abilities to be a bit random. No one really knows why this happens. Well, there are theories. But that would be spoiler territory.