Open Settings
Tap your name
Tap iCloud
Check that iCloud Sync is turned on for the app or service you want
Connect to Wi-Fi
Plug your device into power
Make sure Low Power Mode is off
Make sure Airplane Mode is off
Check that you are signed in with the correct Apple ID
Restart your device
Open the app you want to sync
Leave the app open until syncing resumes
Check iCloud storage space
Update your device to the latest software version
Sign out of iCloud and sign back in if sync is still paused
Check Apple System Status for iCloud issues