Section Numbers won't appear in Table of Contents (InDesign CS3)

I have done quite a few TOC's in the manuals I create but since upgrading to CS3, I have had problems getting the section numbers to appear in the TOC. Usually, if I create the TOC from scratch, it works fine until I have to update the TOC then the section numbers will disappear. In the doc I am working on now, I cannot get the section numbers to appear at all. I've been thru the InDesign Help - any suggestions?