Open Settings
Tap your name
Tap iCloud
Check that iCloud Sync is turned on for the app or service you want
Make sure you are signed in with the correct Apple ID
Connect to Wi-Fi or enable cellular data for syncing
Plug the device into power
Restart the device
Turn the iCloud sync setting off and back on
Sign out of iCloud and sign back in
Update the device to the latest software version
Check iCloud storage space
Verify Date & Time is set to automatic
Reset network settings if syncing still does not resume