There are mainly three causes due to which you might not be seeing lead data added to your MailChimp list.
a) Error due to Form fields not syncing with custom fields in MailChimp
Please check the form field names, especially the ones you’ve used for the custom fields apart from the email field. The email field gets mapped by default. However, any additional field you add apart from the email field, will need to be synced with the respective custom field in MailChimp. The form fields name must be the same as the MailChimp custom field name.
You can refer to an article that will help you sync multiple Convert Plus form fields with those in MailChimp.
b) Error due to Required fields for MailChimp
MailChimp marks a few fields as required. Please make sure you have not marked any custom fields as required. Many a times, users have just one email field on the opt-in form, but have marked the name field as required in MailChimp. In such cases, the entry is rejected and you will not find this lead data stored in MailChimp.
For example, If your MailChimp list contains 3 required field email, first name, and country name. You need to include all these 3 fields to your Convert Plus form in order to send data to MailChimp list. If not, please uncheck them in the MailChimp list, so that they are no longer marked as required. Screenshot
In this case, you can either remove the required conditions form all input fields or make sure that you include all the required fields in Convert Plus opt-in form.
c) Activated Double opt-in feature
The double opt-in feature of MailChimp is enabled by default. This means that the subscribers receive an email with a confirmation link immediately after submitting the form. These subscribers are added to your MailChimp list only if they click on the confirmation link. Since this is the feature at the MailChimp end, you will see contacts added under Convert Plus Connects, but not your MailChimp list.
In case you wish to disable the double opt-in feature and add subscribers to the list anyway, you can do that by enabling the disable double opt-in feature in Convert Plus.
You’ll find this option by going to the Convert Plus Dashboard -> Modules -> Advanced Settings
Here is an article you can refer to.