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For more information on this, take a look at this article:
iCloud: Backup and restore overview
For troubleshooting on this, take a look at this article:
iCloud: Troubleshooting creating backups
- If automatic backups don't work as expected, test to see what happens when you start a manual backup by tapping Back Up Now at the bottom of the Storage & Backup screen.
- Backups and restores both take time, especially if you have a lot of data or a slow connection. Depending on the size of your data and the speed of your network, you may need to give more time than you expected to a backup or restore.
- Make sure you have at least 50 MB of free space available on your iOS device before you back up. If you have no space available on your device, iCloud Backup can fail in some cases. You can verify how much free space you have available on your iOS device in Settings > General > About.
Hope this helps,