unlinked section headers gone haywire

[dra.no...@gmail.com]

I have Word:mac 2008, version 12.3.6. I am editing a document that was created in Word for Windows (version unknown, sent to me by a client, but obviously compatible--at least partially--with mine). The doc is 238 pages long and has multiple sections, and up to the 16th section, there was no problem with unlinking a section from the previous one so I could personalize each section header. However, in section 17, the little checkbox in my Toolbox indicates that it is already unlinked from the previous section, and yet I can't change the headers in this section without also affecting the previous section. In fact, I have now double-checked all sections, and every one is unlinked from the previous one. To top it off, I have apparently also created a section 18, which has the header I wanted for section 17, but I can't find the section break line that starts 18 (and yes, I have all non-printing characters revealed).

Help! What am I missing? How do I fix this?

Deadline is looming, any help is GREATLY appreciated.

lgt