Thank you for the reply wondrack!
What were the original source of the photos that are not syncing? For instance, were they downloaded from a browser or a third-party source? Or were these photos taken from your camera? This can help us isolate the problem and see if there is a common source.
Let's make sure your devices are up-to-date. If it is updated, let's restart your devices. Restarting your devices and making sure they are updated can resolve many unexpected issues.
How to restart your iPhone: Restart your iPhone
How to restart your Mac: Shut down or restart your Mac
Update your iPhone: Update your iPhone, iPad, or iPod touch
Update your Mac: Update macOS on Mac
Keep us posted.