Photos do not sync across devices and iCloud

Are all your devices and the family devices using either iOS 16, iPadOS 16, or macOS 13 Ventura? Otherwise you will no longer see the photos you shared to the Shared iCloud Photos Library on the devices with older system versions.

The first syncing of the shared library will take a very long time. Try to be patient and refrain from toggling the Settings for iCloud. For my library with just 250GB it took more than week, before I could see new synced photos.

Photos needs first to upload all shared items into the Shared iCloud Photos Library, then sync the Shared Library to the subscribers, and only then new photos can be synced from iCloud to the devices. If you disable iCloud Photos after having shared a Shared Library you will have a hard time to set up a shared library again, also all subscribers. Each person can only participate in one shared library. Each of your subscribers would have to rescue their shared photos and leave the shared library before you sign out of iCloud and stop sharing the library.

What you can do while you are waiting is to check for incompatible items in your Photos Libraries on all devices.

If you are having referenced items in the OPhotos Library on your Mac, consolidate them into the library. Referenced items can block the iCloud syncing. Remove any videos or image files using older legacy codecs and convert them to a more compatible format (As a hint which formats to look for: About legacy media in iMovie for macOS - Apple Support).

It helps to share the photos in easy stages, not the complete library at once. Then it is easier to keep track if the items that have been shared and to recognise problematic items.

I just found back to this link: Unread, iCloud shared photo library - can't leave/delete Shared Library

It describes very drastically what a mess it will be for the subscribers to your shared iCloud Photos Library, if the person, who set up the shared library accidentally stops sharing it. Don't do anything, that might accidentally turn off either your iCloud Photos Library or stop sharing the Shared Library.

CrazyAppleMan Said:

"Photos do not sync across devices and iCloud: Hello All, Photos are not synchronizing across my apple devices (iPhone, MacBook, iPad)and iCloud as well. Essentially, photos are not synchronizing across the board.[...]"

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Perform New iCloud Sessions:

Log out of iCloud on all of your devices, and then log back in. Perhaps it all needs to resync?

CrazyAppleMan Said:

"Photos do not sync across devices and iCloud: Let me try that. Shouldn’t cause other problems?"

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No problems would occur. All downloads would reside in Music and TV. You just wouldn't be able to play them, until you are logged in to the same Apple ID on the device.

This may help. Set up or view an iCloud Shared Photo Library on all your devices - Apple Support