Knock your cloud connectivity issues on the head
Apple’s online service keeps a copy of your data in the cloud (meaning Apple’s servers) and keeps it in sync with any changes you make on your Macs
and iOS devices. It’s very clever but, as with all very clever things, there are lots of ways for it to go wrong.
The most obvious reasons for photos, contacts, emails, reminders, calendar items or something else to fail to materialise on a device is that you either haven’t signed in to iCloud on it, you’ve signed in to the wrong account, or you’ve logged in but haven’t turned on the relevant service. Check in > System Preferences > iCloud on your Mac. In iOS, go to Settings > [your name] > iCloud. The Apple ID you’ve signed in with is shown under the account’s profile picture.
If you need to switch to a different Apple ID, you can’t simply move data between accounts. However, when you turn off syncing of Contacts, Calendars or Reminders, you can opt to keep their data on your device. When you sign in with a new Apple ID and turn the service back on, you can opt to merge the data on your device into that account.