Lightroom Mobile stopped uploading to cloud for me. I'm running the current 2.7.3 iOS version. Anything changed on the iPad or iPhone does not upload. If I add a collection, it starts syncing but never stops and nothing appears in the cloud. If I add a photo, to an existing collection, I get the same "syncing" 1 file pending forever. If I delete the additions and quit the app and start again, it will sync fine and show "all up to date".
If I make changes to Lightroom desktop, those sync immediately to the iPad and iPhone. No problem. So the issue is only on the mobile side.
A day later and the problem has disappeared. Must have been an issue with the Adobe Cloud somehow. All good now.