An order was split into 2 shipments. The customer only received an email for the USPS tracking #. They did not get an email with the fed ex tracking #. Why?

These shipments were from your own location. The tracking email doesn't get sent automatically for local locations. You have to send it by clicking the email icon button. The only exception to this is if you use EasyPost to create the shipment and tracking information. If you manually put the tracking information in, you also have to manually send the email.