For those with a  BigCommerce store, there is a default setting in place that prevents people who accept marketing at checkout from automatically receiving the welcome series. A welcome series flow is designed with brand new visitors in mind. These are people who have never before been exposed to your business. People who accept marketing at checkout could be new to your brand, or they could have purchased from you a few times before, which would mean it might not be appropriate for this person to get queued up for your welcome series. 

This said, this setting is completely optional! If it makes more sense for your brand to be sending welcome series emails to customers that accept marketing at checkout, please contact our Success Team to have this setting adjusted.