Out for Delivery and Delivered Notifications

[KarenA22]

Most of our customers are not receiving 'Out for Delivery' and 'Delivered' notifications even though we have the option selected in the Notifications section of Shopify.  We use ShipStation for fulfillment - is that the issue?  When looking over several recent orders, it appears only one individual is receiving those notifications.  I cannot identify anything unique about that custome