Hey there!

You likely have your mailchimp integration set to ‘double opt in’. When you have this setting enabled the visitor needs to confirm their subscription through an email Mailchimp sends out. Otherwise they are never added to your Mailchimp list.

Alternatively you can change your integration settings to a single opt in option and have them added to the list without having them confirm their email. You can do so by cancelling and reconnecting your mailchimp integration.