There can be several reasons why your iPhone might not back up. One common issue is that the backup process is interrupted by a restart, low battery, or slow Wi-Fi connection. If your iPhone is unable to back up to iCloud, it may be because there is not enough storage space available.
To check whether your iPhone is successfully backing up to iCloud, open the Settings app and tap iCloud. Under iCloudBackup, tap the switch to turn it on. If …