Excel 2003 Dropped Links

I have several places where I import data into Excel and use that data to create Pivot Tables.

Occasionally, when I open the Excel file and allow refresh of data, one of the tables does not get refreshed. When I check the linkage, there is none. It is as if the data is stand alone and I have to delete it and redo the import data function. After that it will continue to link and refresh until some future time when it may drop again.