This problem is usually caused when one changes the sync catalog, such as going to a backup catalog, and has become more and more common since v13.3 when they changed the sync architecture. I call these "Orphan" images.
An ‘orphan’ image is where the image is in LrC and may have the icon showing that it is a synced image, and may appear in the “All Synced Photographs” special collection as well as one or more other synced collections but is not in Lr/Cloud. If you un-sync and then re-sync such an image, the sync status shows that it is syncing but as soon as it is done it is immediatly un-synced, renmoved from the All Synced Photographs special collect and from all other synced collections.
One cause of this is when a synced image is present and synced at the time a catalog BU is taken but was subsequently removed from LrC or Lr before the BU was restored. The image then come back into LrC with the restored catalog but it's mate in Lr/Cloud is not there since it had been deleted after the BU was created. This also results in the counts of synced images to be different between LrC and Lr/Cloud.
One solution that sometimes works is to
- save metadata to disk with LrC
- Remove the image from LrC
- Re-import the image
- Re-sync the image
Note: you will lose edit history, you will lose participation in all collecitons, you will lose participation of image in Publish Services.
Editing the image, un-syncing and re-syncing do not work with these orphan images.