Payment Due Reminders will only fire if the project:
Is in an 'Open' state (not Archived).
Has Payment Due Reminders Enabled (Article).
Has a 'Due' Balance in a Signed state.
Note: If the project is still in an 'Unsigned Changes' state, then the project will only receive Payment Due Reminders automatically when the project is signed/marked as signed and the changes have been approved/authorized.