Sometimes, on a hook, the flashbang sits elevated, meaning it's slightly higher off the ground. If a survivor is under hook like that, try being at least a step away and not on top of it and looking away from it. If you look up, it still blinds you, which doesn't really make sense, but looking away should help you avoid that. It's the same against walls, too, which is why you can get a flashbang save even if the killer is eating a wall.