Tokens occasionally display codes that look valid even when upside down. Make sure that
You see only digits (not letters)
The green button is on the left side
The codes that tokens generate can get out of sync with the Duo authentication system. If you keep your token in a pocket or on a keyring - or if you have someone in your life who likes to press buttons - your token may get out of sync because
the button was pressed repeatedly over a short period of time
the token generated numbers that were not used
Tokens can also get out of sync if they are not used for over a month, or if the battery is dying (token batteries generally last between 2 and 5 years).
Customers can attempt a self-service resync of the token by following steps from Indiana University: https://kb.iu.edu/d/bfnf