Been having this same problem since updating to 9.0 way back when. Phone stopped backing up to icloud. I've updated each time a new one came out HOPING it would fix the issue, no luck yet. FOUR MONTHS I have been dealing with this. Several calls to Apple, no help. I have tried all the suggestions on this forum plus every one on the Apple Community Forum, still does not work. I have spent HOURS dealing with this. Some of the agents act like they have no idea what I'm talking about. Some agents will try to help but everything they've done/suggested does not work. I updated to 9.2.1 and the backup still does not work.
The agent told me it was not my device, it was my software, but a different agent told me it was icloud servers, as 9 is on a different server than 8. Ios 8 worked all the time every day no matter what. ios 9 has worked on it's own a handful of times in the past 4 months but will go days without working and then I have to hard reset phone and do a manual backup.
I am being told "It's a known issue" as well, but they have made no attempt to fix it. 9 came out in September and multiple users still can't back up their phones. I don't understand why they release updates before they've been tested determined that they actually work. This whole back up issue is a nightmare for me. I wish I could get back all the hours wasted.
I honestly don't think they care or it would have been fixed for EVERYONE by now.