There can be several reasons why newsletter form subscribers using the embedded form on your Shopify store don't make it to Mailchimp. Here are typical scenarios that you can troubleshoot.
Required merge tags are enabled on your Mailchimp list - when required tags are toggled on, Mailchimp rejects the email because it doesn't include them in the standard newsletter form submission.
Double opt-in is turned on - if you have double opt-in enabled on an audience, you will need to wait until the customers accepts marketing.
Email addresses are from spambots - we've identified that addresses coming through a signup form and not making it over to Mailchimp can be spam. Bots are submitting these addresses to your list. Mailchimp is classifying them as "being added too many times over a set period," and their spam algorithms are blacklisting these addresses for up to 48 hours.
Mailchimp API failures - the Mailchimp API can fail to accept the email subscriber at the time of submission. This could be for reasons beyond our control, e.g., account is in a compliance state, their API was having issues, etc.
Shopify API outages - from time to time Shopify may have API related problems that fail to submit the
customer.create webhook to Mailchimp. You may check Shopify's API status here.