Open the subreport and look at the select expert statement.
you shold see something like:
table.field = ?pm-table.field
the blue section is the field you want to filter on in the sub-report and the red section is the value from the main reprot that was passed in via your linking.
Does the statement in the subreport makes sense as far as both parts being accurate?
as an aside, I don't like to put subreports in the detail section as they run for every row but