This problem occurs randomly and you can see there are files to be synced but they never do unless you stop and restart the whole folder sync again, but the problem then re-occurs.
Microsoft Engineer advised this is the resolution.
Stop the folder sync
Terminate groove.exe and mqsvc.exe
Go to AppData/Locla/Microsoft/Office
For the 16.0 and 15.0 folders.
Then go to Microsoft Credential Manager and clear all Microsoft Office entries.
Restart the One Drive For Business Sync Client and re-sync to sharepoint