I'm aware that you have solved your problem but maybe there was an easier solution. Recently Samsung notified me that OneDrive would be the new cloud service for my photos. I've logged in with my Microsoft account and tried to sync my photos to the new cloud app, but as you said only some photos would be uploaded. After some troubleshooting, I could solve the issue: =>Simply go to your OneDrive account, in settings and sync your media storage. It immediately made my new photos that weren't syncing start being uploaded. I hope this information helps some people that might be having the same issue.
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