On Windows, the status icons for backed up or synced files may disappear. This is because there are only a limited number of slots for such icons in Windows - and these slots can be occupied by other programs.
Check if this is the case by doing the following:
Open RegEdit (Registry Editor)
Copy the following path or navigate to: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellIconOverlayIdentifiers
Check that "JottaExt1", "JottaExt2" and "JottaExt3" are at the top, as shown here: