does Photos update when it's not open?
Does Photos update when the program is closed?
Question marked as
No, it has to be open to sync with iCloud. What does your sync status say at the bottom of the all photos view?
Or are you meaning an "updating" after opening photos but before you can see the images? Have you just updated to Monterey (or other version?)
If so, the library upgrade can take some time - even up to a day or more for a large library, especially if it is on an external drive. Leave your mac on connected to power while it is running.