Unable to upload my Mac Photos to iCloud

I have an old 2009 mac and recently increased my icloud storage to store my photos on. The upload has taken over 4 weeks and still not completed. i have noticed that photos on my mac which show as being edited are not being uploaded but instead being shown under 'unable to upload' why are all the edited photos (and there are thousands) not being uploaded. Can anyone tell me why and how to resolve.

[Re-Titled by Moderator]