Вы обработали событие SubreportProcessing ?
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
AddHandler ReportViewer1.LocalReport.SubreportProcessing, AddressOf Me.SubreportProcessingEventHandler
End Sub
Public Sub SubreportProcessingEventHandler(ByVal sender As Object, ByVal e As Microsoft.Reporting.WebForms.SubreportProcessingEventArgs)
'TODO
End Sub
Вот ссылка на страницу MSDN: нажмите здесь