Auto Opt-In doesn't appear to be working

I'm not sure if there is some sort of cron job that runs periodically to update the MailChimp list with new users, but all the settings in my plugin are set to auto opt-in new users, and it doesn't seem to be happening when users create a new account.

Not a huge issue, as it's no problem right now to just update the list manually from inside the plugin, but it'd be nice if there was just a small thing that I could fix to get this working properly.

Thanks!