When the "Statement Generation" is pressed, this runs a processor that generates and saves PDFs against a contact.
This also updates a field on the Contact called "Last Generated Statement Amount".
When the "Send Statement" process runs, it only updates the Campaign Member status where the related Contact - "Last Generated Statement Amount" is greater than zero.
This is so Statements are NOT sent out when the statement does not have a value or is negative.
Related: Emails Not Sending
If your Campaign Member Status is not updating as expected, or emails are not being sent, this may also be related to the **“Save PDFs”** option selected during processing.
See: Why are my Annual Statement emails not sending after Statement Generate or Statement Send? - This article explains how Campaign Member Status and email sending are linked.