Can't update org chart with added records (ODBC error)

Hi there. I have a Visio (2007) organization chart created with the
wizard using an Excel spread*** and the ODBC method. When I update
the details in the *** and then in Visio choose "Refresh Shape
Data", it works fine. When I add a (valid) record to the Excel ***
(still within the named region) and choose "Update Linked Records", it
takes a while, then gives me the error:
Database Wizard Error
Run the Database Wizard and re-link the shape to correct the problem.
ODBC Error: 23000
[Microsoft][ODBC Excel Driver] Cannot update '(expression)'; field not
updateable.

Can anyone help me fix this?
I need to be able to update the chart to show new people as they are
added and that sort of thing.
.