Jonathan,
this is not an Answer but perhaps it will be Helpful: one thing I have found is that column titles can make configuration really confusing. If given lists have identical or similar column titles, then getting the visual cues to help ensure that the configuration is correct is hindered.
So, for example, in my subcategory list, the field defined in the SP definition as a lookup would have a name like "parentCategory", instead of just "Title".
Having done that, it will be easier for you and for others to tell if there is a problem with the configuration.