Consider not putting a parameter in the subreport, rather pointing the main report parameter to the fields in the subreport.
In either case, you can verify what is being used by the subreport by right clicking the subreport and selecting eidt subreport, and then check the Report->Edit Selection Formula->Record
You can also verify what is being passed to the database by ckecking the Database->Show SQL Query in the subreport.
-k