I've been working on this issue for about a week, all started when i installed 9.3b on my iPhone, i don't think its related, but noticed 9.3b wasn't backing up to icloud even though it was enabled, first thing i tried was delete previous backups in icloud, turned off backup and after that wouldn't turn back on. Thought it was 9.3 issue, so i restored back to 9.2, backup still wouldn't turn back on. Then when on my iPad air, noticed hasn't been backup to icloud since dec 6th, manually forced backup but failed like my iPhone, turned backup off and after that wouldn't turn back on either.
I have been working with Apple support, one thing we noticed was that their test device was able to backup, however after deleting the backup from the icloud, would not backup anymore. Also noticed just on my iPhone, none of my previous icloud drive data was showing, only new items i add will sync over to my iPhone, this wasn't the case for the iPad, was working correctly.
I tried everything i can find on the net with no success. My guess is the backups are not actually getting deleted in my icloud storage, its some how still sitting there, even though apple support says they can't see any backup data on their side. Most likely my storage is corrupted, hoping apple can partition a new storage for me and migrate over. Waiting for call back from apple engineers, i will update after.