Why is my Shopify welcome-email automation not sending to customers who already show as subscribed?

That’s usually expected: Shopify’s welcome-new-subscribers automation is looking for a new qualifying signup event, not every customer who already says Subscribed. In particular, Shopify says that trigger works for a form on your online store, not third-party form subscriptions, and its welcome templates exclude people who subscribed at checkout. So check how the customer opted in, whether they subscribed before the automation was turned on, and whether they came through checkout. Then test with a brand-new email using the exact signup form you expect to trigger it. Also make sure the automation is actually turned on and there isn’t an error shown on its Automations page.