If you recently noticed a tiny message at the bottom of the Photos tab in OS X’s Photos stating that it was unable to upload a number of items, you aren't alone. I have this issue, and I’ve heard from others that have encountered this as well.
But what are the items that Photos is unable to upload? You can't simply click on the message to view the items (although that should be the case). After trying a few things, I was surprised to find "unable to upload to iCloud Photo Library" as an option when creating a Smart Album.
To make a Smart Album that contains all the items that Photos couldn't upload to iCloud Photos Library:
- Under the File menu in the Photos app, select "New Smart Album..."
- Set the conditions to "Photo", "is" and "unable to upload to iCloud Photo Library"
Once you have created the Smart Album, the troubleshooting begins. As a start, select one item at a time and press Command-i for an information pane. See if the items have something in common.