Are you running the latest Lr Desktop 5.6 version. If not I would recomment to update since we have fixed lots of sync issues with this version.
If you alreday use version 5.6 try to remove the sync.lrdata file and restart LrD. This would restart/refresh the sync (only metadata).
Removing the Sync.lrdata might fix whatever the issue is. Or, at least, might trigger it to occur again. So remove the Sync.lrdata, start Lr deskktop, and then wait if that helps. If not, create at this point a Lr diagnostic log which can hopefully provide some clues.
You can find the sync.lrdata file here:
On Mac it's at /Users/<user>/Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata
On Win it would be at C:\Users\<user>\AppData\Local\Adobe\Lightroom\Caches\Sync Data\Sync.lrdata
You can trigger the log via LR Desktop preferences -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button
- Guido