iCloud Photos generally works to sync images and videos across your hardware linked with the same Apple ID and for which you have Photos syncing enabled. When it doesn’t, don’t expect much feedback from macOS. First, try other debugging advice:
• Quit Photos and launch it while holding down Command and Option, then click Repair. When complete, see if syncing starts.
• Try disabling iCloud Photos in Photos > Settings/Preferences > iCloud. When prompted, select Download and Keep Originals. Then re-enable. This can take hours to days to finish re-syncing if it works.
Read more at Macworld.com