Pivot tables that "lose" data
 

I'm working with a very large workbook (134K lines spread across 20
worksheets with 25 columns of data, including equations). One field/column
is "Type". Some lines have a "Type" value and others are blank in this
field. I have a separate spreadsheet which has pivot tables for each sheet
in the workbook. The pivot tables summarize by what's in the "Type" field.

If I create a copy of the source workbook in order to change the
calculations I'm doing (yet retain the results of each version that I run),
and I reference this new workbook in the pivot tables of the summary
spreadsheet, some of the pivot tables "lose" the values in the "Type" field.
When I choose the "show details" aspect, the "Type" values are gone.

I need to know what might be causing this loss of the "Type" field values
and how I might keep the problem from occuring. I'm using Excel 2007.