For no apparent reason, the Photos app on my Mac stopped syncing with iCloud sometime over the last two months, and neither of the better threads on Apple Support (if your iCloud photos are not syncing) nor on Reddit (Photos on Mac not syncing) worked for me. Here’s what I did that ended up doing the trick:

1. Quit Photos.
2. Open Finder and go to /Users/(your name)/Pictures/Photos Library.photoslibrary
3. Delete that file!
4. Open Photos again.
5. Go to Preferences and select Make New Library.
6. Make it your System Photo Library.
7. Switch to the iCloud tab in Preferences and make sure iCloud Photos is checked.
8. If you have Optimize Mac Storage selected, go back to Library and see if it’s actually updating/downloading from iCloud or if it’s just kinda stuck.
9. If it’s still stuck, try this: go back to the iCloud tab in Preferences and select Download Originals. Go back to Library and see if anything’s changed. Don’t worry, we’re not actually going to download all of them—this is just what jumpstarted Photos into actually syncing for me.
10. Is Photos actually syncing? Yes! Go back to the iCloud tab and select Optimize Mac Storage again.

At this point, all my thumbnails were finally downloading and Photos was once again syncing with iCloud. Hope this helps!