I had the same problem - of my 7000 photos and videos - the photos app was unable to upload half of them!
Have spoke to 5 apple agents this week, all clueless! Just kept asking 'have you got enough cloud space?' Or 'have you turned iCloud on?' FFS
This solved my problem (trial and error NO thanks to Apple)
Create a smart album, criteria should be 'unable to upload to cloud'. Then you have an album with all the dodgy photos! You can check if there are any corrupted ones and delete as necessary. There was nothing wrong with any of mine so...
Then I copied them into a folder on my desktop and deleted the ones in the photos app. Remember to delete your 'deleted photos' too in the photos app.
Then copy all the photos in the desktop folder back into the photos app. This took hours!
Then they miraculously all started uploading!
I now have everything backed up in the iCloud.
Can't believe nobody suggesting doing this! I even had to book a call with a senior apple agent. It was booked for 11am GMT and after 11 hours of waiting, I gave up waiting!
Hope this helps anyone having the same problem!!!