Try this:
1. Delete the existing subreport link.
2. Create a new param in the subreport that is set to multi-value.
3. Create a new subreport link. In the "Subreport parameter field to use" table, select the parameter you just created.
This will ensure that the target param in the subreport is multi-value so it should contain all of the values from the main report.
-Dell