Chapter numbering in composite documents

In order to publish my dita bookmap as HTML format, I chose to save my dita bookmap as "Composite Document 2019 (*.fm)", then publish it as Responsive HTML5. Generally, it works well except for one issue. That is the chapter numbering, which starts from 1 in every chapter in the composite document, although under Numbering Properties, I have set "Continue Numbering From Previous Chapter in Book" and in the Paragraph Designer under Numbering I have "C:<$chapnum>." for main headings and "C:<$chapnum>.<n+>."  for sub-headings. Is there anything else I am forgetting?