Adam Engst’s aforelinked piece on iCloud Photo Library problems reminded me of this piece I wrote two years ago, that might still be of use to anyone who encounters this bug:
I’ll offer a small personal anecdote. Overall I’ve had great
success with iCloud Photo Library. I’ve got over 18,000 photos and
almost 400 videos. And I’ve got a slew of devices — iPhones,
iPads, and Macs — all using the same iCloud account. And those
photos are available from all those devices. Except, a few weeks
ago, I noticed that on my primary Mac, in Photos, at the bottom of
the main “Photos” view, where it tells you exactly how many photos
and videos you have, it said “Unable to Upload 5 Items”.
Restarting didn’t fix it. Waiting didn’t fix it. And clicking on
it didn’t do anything — I wanted to know which five items
couldn’t be uploaded, and why. It seems to me that anybody in this
situation would want to know those two things. But damned if
Photos would tell me.
Eventually, I found this support thread which suggested a
solution: you can create a Smart Group in Photos using “Unable to
upload to iCloud Photo Library” as the matching condition. Bingo:
five items showed up. (Two of them were videos for which the
original files couldn’t be found; three of them were duplicates of
photos that were already in my library.)
I haven’t run into this problem again and am now up to 25,000 photos and just under 900 videos.