I'd be asking the parents if anything has changed at home. I nearly always, if not always find that something has happened at home when there is an unexplained change in a child's temperament. Sounds like they are missing a parent hence why you aren't able to soothe her and if she has been with you a while then it likely isn't separation anxiety. That being said, if something at home has resulted in clinginess with this child and the environment is less than ideal, it could be enough to have this child be a little emotionally unstable. Hopefully a chat with her parents will reveal something and the parents can work on restoring stable home environment to make the child feel more secure....sorry I've no other suggestions.