Hi, hopefully this will help,
Using a MFi cable-
I would backup 'encrypted' via itunes saving to your computer.
Then delete the 'old' iPhone icloud backup on icloud via iPhone (worth making a noteof the 'latest backup')-
iOS
- Tap Settings > iCloud. If you're using iOS 8 or later, tap Storage > Manage Storage. ...
- Tap the name of your iOS device.
- Tap Delete Backup. When you're asked to confirm, choose Turn Off & Delete to turn off Backup and remove all backups for that iOS device from iCloud.
Then try backup 'over the air' again, plugged into mains. I'm hopeful this will work.
If not next try backup to iCloud via iTunes.
It sounds like the iCloud backup has a glitch (I have had this). My iPhone wasn't backing up 'over the air' for a month without my knowledge!
I check daily now backup has been time stamped.
ps
I have a 'reminder' set to back up my iPhone & iPad 'encrypted' via iTunes to my TC, monthly.
Then a further reminder to backup TC to a USB drive that I keep at a relatives house (offsite backup).
So I have three iPhone/ipad backups hopefully that covers all the bases.