Any idea why .csv file will load using manual Import Data but not with Data Loader?

I have a .csv file I'm using to insert Opportunity records.  I have it working fine via the manual Import Data tool.  (Which allows me to pick 'On Demand Predefined Fields' as the unique record matcher.)  However, the Data Loader keeps failing when I use the same .csv and .map files.  I suspect it's because of the unique record matcher because my error is: "*Oracle Data Loader On Demand Import validation FAILED: Some required fields were not mapped. In your mapping file, use the Oracle CRM On Demand fields which correspond the following UI fields: External Unique ID".  But... I'm