If you have added the Mailchimp integration to an experience and the data you are collecting isn't passing through, there's a good chance you have a field marked as "required" in Mailchimp that's not on your Audience form.
To check this, go to the list that you are hoping new contacts get added to. Then click on the settings dropdown and click "List fields and *|MERGE|* tags.
Once there, view the "Required?" column to see if any fields are marked as required. Uncheck the ones that are. As a best practice, don't make any fields required in Mailchimp to prevent this from happening in the future.
If you've already received entries into your experience, you may need to download its CSV file and manually import those people who have already opted in prior to making this correction.
If you have any further questions, don't hesitate to reach out to support@audience.io via email, chat, or phone (801) 901-2011.