I have selected "No, do not display the email opt-in checkbox. Opt-in constituents automatically" but I noticed for some constituents, it didn't opt them back in. Why isn't the survey opting them back in?
You can only opt-in constituents automatically if they are new to your site and haven't already opted out. If a constituent has made a choice to opt-out, then they can not be opted back in without asking for their explicit permission to do so. To add an option to allow a constituent to opt themselves back in, choose the option: "Yes, display the email opt-in checkbox." in the Registration Question.