To rule out a few things.....
Does Classic work normally apart from the sync issue?
Are you signed into Lightroom Mobile?
Is Sync Paused?
You probably have checked all of these but some of this may be helpful if not....
In you desktop version, open the Catalog panel and delete any images showing up in Syncing errors. You won't be removing them from the catalog and re-syncing will initiate. You may have closed the app before syncing was completed...
Syncing error images is not present in Catalog panel. I find that photos synced is 12,394 in Lightroom CC and the Server in my acoount, but all synced Photographs is 12,269 in catalog panel in Lightroom Classic CC. The different between them 125 photos, is that I have exported to Lightroom CC by my iPad. I think all photo is already in Server in my account, but Lightroom Classic CC could retrieve the photos. Network is not in trouble.