Says '*This email already subscribed!'

I had a Mailchimp subscriber that I deleted and archived and they want to subscribe again to get the promo, but when they put their email into the pop-up box it says ‘this email is already subscribed’. When I search for their email in my mailchimp audience it’s not there. How do I force the plugin to sync with the mailchimp audience data?

Alternatively, is there a way I can manually send an email from the WCB to one subscriber?

Thanks!