shared album not updating

Have you been using Shared albums frequently? If we are using them to much, we may have to wait a bit until we are allowed to use them again - there are limits to the number of shared albums, to the number of total photos, and the number of hourly and daily uploads, see: My Photo Stream and Shared Albums limits - Apple Support

The day before yesterday there have been maintenance works on Apple's cloud servers, causing problems with Shared albums. These problems have been fixed by now. Have you tried to disable the shared albums, then restart the Mac, and turn on Shared Albums again? If you do that, save all photos from your shared albums, that you have not yet saved. Older photos, that have been removed from the hared albums may not come back, if you toggle shared albums on and off.