Hi Fred,
If there is a similar parameter in the Subreport (one that you created manually) then you need to make sure this parameter is linked to the parameter from the Main Report.
Right-click the Subreport and go to Change Subreport Links. Could you check if the Parameter from the Main Report is linked to the prompt from the Subreport. The prompt from the Subreport can be selected from the drop down that says 'Subreport Parameter field to use'.
-Abhilash