Hey there!
Good question, off the bat, thinking a few causes. But it may take time and effort to figure out.
Anyhow, Do you know if you have more than one Photos Library(s), in your Home/Pictures folder?
Also, do you happen to store your Photos Library on an external hard drive? Or just on your Mac?
You’d first want to make sure the library you’re trying to open is set as the System Library, as iCloud will only sync to that library:
Designate a System Photos Library:
https://support.apple.com/en-us/HT204414
You May have already tried quitting and relaunching Photos, and restarting your computer, even restarting your router, or disabling iCloud Photos in System Preferences > Apple ID > iCloud, restarting your Mac and enabling it again maybe a good step.
I’d say if still having troubles, you can quit Photos, hold the Option key while re-opening Photos, and re-select your Photos Library, then give it some time.
If that eventually fails, you can attempt to repair the library in case it’s got damaged somehow:
Quit Photos, relaunch while holding Command and Option and select the option to Repair the library. This may also take some time.
Although if the Photos Library are stored externally and not on you Mac directly, there maybe another solution, let me know!
Repair your Photos Library:
https://support.apple.com/en-us/HT204967
Hope any of that helps.
If not, it maybe worth a try creating a new Photos Library, making that the System Library, as in one of the above articles, and see if it syncs there, it could be a corrupt library:
Hold down the Option key while launching Photos and choose, “New Library”, choose a location and name, the set as default/system in Photos Preferences, and test.
Id say if those failed and was given sufficient time, and load on your other devices fine, trying another network is a good idea, but disconnecting your router/modem from power for a minute/re connecting it, and waiting is also an easy step, although it may connect with other devices fine, I have seen this be isolated to one device on the same network before.
However I’ve had luck with the previous steps listed in the past, so hope those help!