Roy,

If they are clicking the link and completing the authentication process on the site / module. It is very unlikely that they are not been added. Remember that unsubscribe is global, so they may not be receiving email messages as they are on the unsubscribe list.

Or it may be going to spam, depending on a range of servers setups.

You can check in the campaign and search for an email address to see if the module sent a message to them.