Probably the Tabbles overlay handler is working, but windows is not showing the overlay icon because you have other overlay handlers installed, with greater priority.

Open registry editor, and go to the location:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellIconOverlayIdentifiers

You should see a key named 000TabblesOverlayHandler

If there are other applications before, they are getting precedence over Tabbles.

This is a Windows limitation. You cannot have two applications setting an overlay on the same file. Only one can do that, the others are ignored by Windows. Which one will be able to set the icon, is defined by the order in the registry.

You can either rename the tabbles key to gain precedence, or uninstall the other applications.

Let me know if my diagnosis is correct.