First remove the offending photo that doesn't finish syncing from the desired Collection(s).
Use the Hidden "Rebuild Sync Data" Button
- In Lightroom Classic, go to Preferences (Mac:
Lightroom Classic > Preferences| Windows:Edit > Preferences). - A hidden Rebuild Sync Data button will appear in the window.
Click it. - Lightroom will prompt you to restart and rebuild. Let it finish (this might take some time depending on your library size, but it is highly effective).
- Click on the Lightroom Sync tab.
- Press and hold the Alt key (Windows) or Option key (Mac) on your keyboard.
Then the "Clean Slate" DNG Export (The Ultimate Fix)
If the file container itself has a tiny bit of data corruption, Lightroom will read it fine locally but fail to package it for the cloud. Exporting it as a DNG creates a mathematically perfect, uncorrupted copy of your photo with all your edits baked in.
Select the photo and press Ctrl + Shift + E (Windows) or Cmd + Shift + E (Mac) to open the Export dialog.
Under Export To, choose a temporary folder (like your Desktop).
Under File Settings > Image Format, select DNG.
Click Export.
Import this brand-new DNG file back into Lightroom Classic.
Add this new DNG version to your synced collection (it will sync flawlessly), and you can safely delete or archive the original buggy file.