The S80 tag is essentially grabbing variables from a user session. Since the Thank You for Registering autoresponder is connected to the registration, the S80 tag is able to pull the TeamRaiser ID variable out of the user session. When the follow up autoresponder is sent, it is no longer connected with that user's registration session and so it has no variables to pull from. Instead of using the S80 tag in your reusable page, you should instead use [[S42:0:fr-id]]. This S-tag will recognize which TeamRaiser it is being sent from and will render the TeamRaiser ID without relying on session data.