Sage History Detail

I do a SQL join of the SAGE_HISTORY_HEADER and SAGE_HISTORY_DETAIL DB tables by the column NUMBER, but I have some HEADER records that don't have any matching DETAIL records.  When I look them up via the application and drill down into the order (invoice), I can see the items.  Does anyone have any idea why this might be and what I need to do to pull the DETAIL items at the SQL query level?