Sorting function not called in Flex Datagrid

I am creating a Datagrid in AS3, and adding a sort function to a column. However, this sort function does never get called. Any ideas?

            dg_gruppenUebersicht = new NestedDataGrid;
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            dg_gruppenUebersicht.dataProvider = arrCol_gruppenTnAkt;
            dg_gruppenUebersicht.columns = new Array;
            var cols : Array = dg_gruppenUebersicht.columns;

            //initalize datagrid
            var dgc : DataGridColumn = new DataGridColumn( "Jahrgang" );
            dgc.dataField = "TnJahrgang";
            dgc.labelFunction = MyFormatter.lblfunc_alter2jahrgang;
            dgc.sortable = true;
            dgc.sortCompareFunction = compareFunc_tnJahrgang;
            dgc.headerText = "Alter";
            dgc.headerRenderer = new ClassFactory( at.moschitz.VDGHeader );
            dgc.width = 20;
            cols.push( dgc );

            dg_gruppenUebersicht.columns = cols;

Strangely, the label function gets called.

Thanks Martin

Try calling arrCol_gruppenTnAkt.refresh(); at the end.

Are there other columns in the data grid? If so, did you click on the dgc column so that the grid is sorted as per this column?