iCloud Sync issues
iCloud sync is usually reliable, but sometimes you’ll find content fails to sync between all your devices in the few seconds it should take. If this seems to be happening to you, these simple tricks can help get things running smoothly again.
About Activation
Before the app syncs data, iCloud Sync must be enabled in the app settings. iCloud Sync needs a Premium subscription / purchase. If you can't active the toggle, check your subscription status.
After activation you need to force close the app and restart. Then it might take some seconds or minutes to active and sync data.
Check that iCloud is working in General
It also makes sense to double-check that iCloud services are working correctly before running through any other changes. You can visit Apple’s System Status page to make sure things are indeed up and running.
Make sure you are properly logged in
Step two is to ensure you are logged into iCloud using the same Apple ID on all your devices. Go to icloud.com, login with your Apple ID and then tap iCloud Settings (either the gear-shaped box icon or by selecting it in the drop-down menu underneath your name at the top right of the iCloud browser window).
In the next window, you should see your storage space status and a row called My Devices. Are all the devices you want to sync included on the list? If not, it is possible they are not using the same Apple ID.
Check dates and time
Next, check all the devices that should be syncing. You must ensure these are configured to set time and date automatically and have iCloud Drive/Documents & Data enabled. Follow these steps:
iOS: Settings > Apple ID > iCloud > iCloud Drive. Toggle to On
Log out of iCloud and log in again
If you regularly experience sync problems with your iOS device(s) and you know your network is stable, then you should try logging out of your iCloud account on your iOS device or Mac, then log back in.
Restart your device(s)
If problems persist then close and restart the relevant iCloud-enabled app: Contacts or Calendar, for example. Double-click the Home button (or swipe up to about halfway up the screen and hold for a second or so on iPhone X), swipe through your active apps and swipe up to close the app. (You can long press the app icon and then tap the X that appears on iPhone X.) Return to the Home screen and wait a few moments before launching the app again.
Another approach that sometimes works is to turn off iCloud Contacts and turn it on again. Go to Settings>Apple ID>iCloud, then turn off Contacts. Unless you have a copy of your contacts stored elsewhere, you should then choose Keep on My iPhone/iPad.
Wait a few moments and turn Contacts on again in Settings.
Please give activation and first sync some time. It will start to sync :)