Since I upgraded to iOS 9, iCloud backup is not working anymore. I managed to complete one backup after I deleted all the old backups and probably pushed the back up now button for over two hours until eventually one backup was completed. Since then, no automatic update could succesfully be completed. This was a week ago. Googling the issue returns results of many people experiencing the exact same thing. I had initially the same problem on my older iPhone as well but it's working now. I can't get it to work on my iPhone 6 though. When you go to backups, it will say that the next backup will be 0kb.