Crossreference Errors in Book File

I have a book file that includes 5 documents with cross-references. I can set up the cross references in each individual file without issue, but once I try to export the complete book to PDF, I get an error message that I have invalid cross references.

Sure enough, when I go back into an individual file, I'll have to re-link some of the cross references, only to have them break all over again upon the next export attempt.  I can't figure out what's going wrong, there doesn't appear to be much consistency in which references break each time either.  Any advice would be greatly appreciated.