Library on Photos not syncing correctly with iCloud

Hello jmaggi,

This page has some info and steps that may be able to assist: Check the status of your library in Photos for macOS

Find library status information in Photos

1. Select Library in the Photos sidebar.

2. Click All Photos in the toolbar at the top of the window.

3. Scroll to the bottom of the window to see status information:

•The number of photos and videos that are in your library.

•When your library was last updated with iCloud Photos, and the option to click Pause or Resume if updates are in progress.

•The number of items that couldn't sync to iCloud. Click View for more details.

•The number of items — if any — that are on this Mac only and not available to your other devices.

If items couldn't sync to iCloud

To see which items couldn't sync to iCloud, click the View button next to the message at the bottom of the Library window or click Unable to Upload in the sidebar. You can try to reimport the items from the Unable to Upload album into your Photos library to sync the items with iCloud Photos. 

1. Press Command-A to select all of the items in the Unable to Upload album, choose File > Export, then choose Export Unmodified Original For [number] Photos.

2. In the dialogs that follow, click Export, choose a save location, then click Export Originals.

3. After the items export, make sure they are all still selected in the album, then choose Image > Delete [number] Photos or Videos.

4. Click Delete, then click OK.

5. Choose the Recently Deleted album in the sidebar, then choose Delete All. Or delete only the items that you just deleted. 

6. Choose File > Import, then choose the images you previously exported. For any items successfully synced to iCloud Photos, edits or keywords that you added to the affected items might be lost.

If you see "Items on This Mac Only"

If you set up Photos to allow referenced files, which are photos and videos stored outside of your library, Photos counts those as “Items on This Mac Only.” If you want to consolidate those items into your library so that they can be uploaded to iCloud Photos and available to your other devices, follow these steps:

1. From the menu bar in Photos, choose File > New Smart Album.

2. Name the Smart Album “Referenced” and set the conditions to “Photo is referenced.” Click OK.

3. Select the Referenced album in the sidebar.

4. Press Command-A to select all items in the album, then choose File > Consolidate.

Cheers.

jmaggi wrote:

I tried to disconnect iCloud on my MacBook's Photos app and reconnect it, it synced a few additional pictures, but it did not solve the problem.

Just a caution - I would not do that, unless it is absolutely necessary as a last resort. When you turn off iCloud Photos on your Mac, the complete Photos Library will need to be uploaded again to compare it to the items already in iCloud, as your Mac will treat your Photos Library as a new library that needs merging into iCloud. It is a very expensive and time consuming fix and causing a lot of network traffic. You may not even be able to turn iCloud on again, if you do not have enough free cloud storage for a second copy of your library.

Try to find out, which photos or videos are missing in iCloud with the help of the status bar as shown by Eric_G1 .

You have to find and remove any items shown as "Unable to upload" or "On this Mac only", as one bad apple could be blocking the upload of further items, that would be compatible.

Most likely some of your videos need to be converted to another, more compatible format. This should have been done before upgrading from macOS 10.14 Mojave to macOS 10.15 Catalina.

You can test your videos by exporting them from your Mac with "File > Export > Export unmodified original" and then try to reimport them to a new, empty library. Photos will then give a warning, if you try to import an incompatible video. Any video that can no longer be imported should be removed from your Photos Library and converted to a different format, for example with HandBrake.

After the upgrade to Catalina I had also to remove some image files in an older format, also all PDFs and Audio files that had been carried over from old iPhoto and Aperture Libraries.

This document for iMovie can give you an idea, which codecs will no longer work on macOS 15 Catalina: About legacy media in iMovie for macOS - Apple Support

If the status bar does not show you any "unable to upload" items and there are no "on this Mac only" items, use smart albums to search for items in a legacy format, for example like this: How to Weed out Legacy Media in Photos for Mac - Apple Community