I think there is a problem with emails that unsubscribe then subscribe again.
I have used a couple of my emails address for testing and playing around with list...at the end I have unsubscribed all then I have added them again.
I receive the confirmation and the welcome messages but I do not receive any follows ups. The follow up works fine for any other new email address but not for an email address that was unsubscribed and then subscribed again.
Maybe you can check this.