Back up and iCloud storage
Why is that I am having problems backing off my devices even though I still have space left on the cloud?
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There are a couple of really basic suggestions in here that might help.
If you can't back up to iCloud or restore from iCloud backup failed - Apple Support
If your iCloud backup stalls or will not complete the backup, try quitting the backup, force close the settings app, turn off WiFi, restart your device, turn WiFi on and then try to backup again.
If you can’t quit the backup and you get the spinning wheel and it never ends, just force close the settings app, turn WiFi off, restart, turn WiFi on and then try the backup again.