The photo library on my mac last updated 3 days ago.
What'd does the status line in Photos on your Mac show? Please check, if there are photos on your Mac that cannot upload to iCloud Photo Library, see: Check the status of your library in Photos for macOS - Apple Support
If there are items in the smart album "Unable to upload", you have to remove them from your Mac Photos Library. The Mac will not download anything from iCloud Photo Library, before it uploaded all pending items.
The High Sierra upgrade introduced a new problem with the syncing of iCloud Photo Library. You need a Wi-Fi connection for the Mac. A connection to a cellular Hot-Spot does not suffice to sync the Photos Library. And in some cases the Mac did not longer sync with iCloud Photo Library, even o ver a fast ethernet connection, if a VPN has been enabled. Some older VPN software needs updating to work with High Sierra.