Word multi level lists

I'm having a problem getting the Word multi level list function working properly. I have a legal document with Articles and Sections, I have managed to get the Articles to number correctly, and the first section of each article to number OK, but the subsequent sections under that article keep the subsection number 1. For example, I get

Article IX

Section 9.1
Section 9.1
Section 9.1

I have checked all the heading descriptions, and they appear to be OK, i.e. Heading type 1, 2 etc.

How can I get the sections to number correctly? I've tried with both Word 2003 and 2010.