TOC book in parent doesn't expand (OK from subproject, OK in past)

I have a merged project. The TOCs of the subprojects work fine if I open the webhelp from the subprojects (...mergedProjects/subproject..), but if I open the project from the parent, although the TOC seems fine at first sight, when I click on a book in the TOC it does not expand as it should. It all worked fine until yesterday (I think)...

I don't remember changing anything significant between builds - though clearly I must have!

How do I fix this?

Thanks

Peter