What does the status bar below the Photos view show?
You may want to check for items, that cannot be uploaded to iCloud, with the two smart albums described here:
About the status bar in Photos for macOS - Apple Support
Unable to Upload [#] Items appears if there are any photos or videos on your Mac that couldn't be uploaded to iCloud Photos. You might be able to fix this with steps below, but edits and keywords that you applied to these photos will be lost.
- Create a new Smart Album by choosing File > New Smart Album.
- Name the Smart Album "Unable to Upload" and set the three pop-up menus to Photo - is - unable to upload to iCloud Photos.
- Select "Unable to Upload" in the left sidebar.
- Press Command-A to select all the photos in the Smart Album, and then choose File > Export > Export Unmodified Original For (#) Photos.
- In the dialogs that follow, choose the file name format and location for the exported photos. Click Export, then click Export Originals.
- After the photos export, make sure the photos are all still selected in the Smart Album. Then right-click or Control-click one of them, and choose Delete (#) Photos from the menu that appears.
- Click Delete, then click OK.
- To import the photos back into your library, choose File > Import and find the images in the location you chose in step 5.