I think budgies in general on their own don't really do well. Unless, that's how they choose to live their life. FOr instance, they were aggressive so they got seperated and they do better on their own now. From what i understand, this seems to not be the case for sky. She seems to be doing better with a pair. I would highly suggest getting her a friend. I have also a similar problem. I have a total of three birds. Two of my birds are bonded but the other one is sort of left out because she doesn't have a pair, even though they live in the same cage, i do find her on her own playing with her mirror and honestly it looks a bit depressing. Due to this, i do find her a bit lonely because even though my other two pairs get along with her, she has no one to preen her or chat with her. In a few months i will be getting her a beautiful male birdie.