If the double opt-in has taken too long (longer than 2 days), the email confirmation will no longer be recognised. The contact must be reset and go through the campaign again - either by signing up again and then completing the Double Opt-in straight away, or by manually pushing the contact into the campaign.
Another reason may be that the contact has an email status that prevents them from being contacted (unsubscribed, blocked, temporarily blocked...). You can find out more about email status here.