If you've sent out an email using Soapbox Mailer, you might find that your Campaign Members are not having their Unsubscribe checkbox checked after they click the unsubscribe link. Here's a few steps to troubleshooting your way towards a solution.
1) Ensure that your Campaign Member Type is set to "Soapbox Mailer"
2) Ensure no validation rule exists on either the Campaign or Campaign Member records that would stop either the Campaign or Campaign Member records from being edited