You've installed the add-in, but the signature doesn't appear or only appears when you click on the add-in? This means that the installation procedure has not been followed.
To correct this, check one thing:
Click on "..." then "Get add-ons".
Web version :
Then go to my Add-ins :
If your installation is correct, the Add-in should be in "Managed by administrator".
Good installation :
Wrong installation :
If your problem persists, don't hesitate to contact support via live chat.