When you open up Lr Desktop preferences-> Lightroom mobile panel....do you see files listed under the pending sync activity window?
Could you try to remove the sync.lrdata file and restart Lr Desktop. This would restart/refresh the sync (metadata only).
Removing the Sync.lrdata might fix whatever the issue is.
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
Hope this helps.
I'm having same issue. Nothing in sync activity. Removed Sync.lrdata folder. May I send you log to remedy this issue?