Do you have temp tables in the stored procedure? Generally there is a temp tables problem due to Microsoft's bug with reading the Metadata. You can find more information on this subject and suggested solution in the following forum thread: Temporary Table in Stored Proc.
Additionally make sure that you have set a valid design time parameter
value according to the stored procedure requirements. Specifying design time values for the parameters is necessary because the designer might need to execute the SQL query or stored procedure of the data source in order to populate the schema displayed in the Data Explorer tool window and the Edit Expression dialog. These values do not affect the execution of the SQL query at runtime however.
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