Photos not syncing

First, confirm that My Photo Stream is enabled on your Mac by going to the Photos app, then Photos > Preferences in the menu bar. Click the iCloud tab and ensure the My Photo Stream box is checked. Second, confirm the same on your iPhone or iPad by going to Settings > iCloud > Photos. The My Photo Stream option should be turned on there. If both of these are already set correctly, follow the steps below:

1. On your Mac, quit the Photos app.
2. Launch the Activity Monitor app, which is located under Utilities in the Applications folder.
3. In Activity Monitor, search for “photos” in the upper-right to filter through the list of running processes.
4. There should be a few rows left on the screen. For “Photos Agent” and each of the ones starting with “com.apple” click on the row to select it, and then click the X icon in the upper-left.
5. A confirmation message will appear. Click “Force Quit” to end the process.
6. Once all of the processes mentioned in step 4 are gone, you can quit Activity Monitor.
7. Open the Photos app again and, after a few moments, all of your latest photos should start importing from My Photo Stream.

From My Photo Stream not updating in Mac Photos app