Jquery datatables serverside fixed header not working

oTable.DataTable({
          //"fixedHeader": {
          //    header: true,
          //},
          //"bLengthChange": false,
          //"Filter": false,
          //"Info": false,
          "bPaginate": true,
          "lengthMenu": [[300, 500, -1], [300, 500, "All"]],
          "dom": 'lrtip',
          "bRetrieve": true,

          "bDestroy": true,

          // "processing": true,
          //"serverSide": true,
          "ajax": {
              "url": "../Dispatch/DispatchTable/" + url,
              "type": "POST",
              //"datatype": "JSON",
              "dataSrc": "",
              "error": function(xhr, status, error) {
                  alert("Use date range");
                  FCol
              }

          },

          "columns": [
              {
                  "data": "FCol",
                  "className": "hidetd"
              }, {
                  "data": "ResStatus",
                  "fnCreatedCell": function(nTd, sData, oData, iRow, iCol) {
                      $(nTd).addClass("rc" + oData["ResStatus"] + " text-center");
                  }
              }, {
                  "data": "FarminID"
              }, {
                  "data": "ResNoLive"
              }, {
                  "data": "Confirm",
                  "className": "text-center"
              }, {
                  "data": "VendorName"
              }, {
                  "data": "ResVenAcptCode"
              }, {
                  "data": "VendorDriver"
              }, {
                  "data": "DispStatus",
                  "fnCreatedCell": function(nTd, sData, oData, iRow, iCol) {
                      $(nTd).addClass("rc" + oData["DispStatus"] + " text-center");
                  }
              }, {
                  "data": "InhouseDriver"
              }, {
                  "data": "RunType",
                  "fnCreatedCell": function(nTd, sData, oData, iRow, iCol) {
                      $(nTd).addClass("rc" + oData["RunType"]);
                  }
              }, {
                  "data": "VehicleName",
                  "fnCreatedCell": function(nTd, sData, oData, iRow, iCol) {
                      $(nTd).addClass("rc" + oData["VehicleCode"]);
                  }
              }, {
                  "data": "DispatcherNotes"
              }, {
                  "data": "DispatchTime",
                  "className": "text-right"
              }, {
                  "data": "DispatcherPickpTime",
                  "className": "text-right"
              }, {
                  "data": "ServiceAreaName"
              }, {
                  "data": "PickupDate"
              }, {
                  "data": {
                      _: "PickupTime",
                      sort: "PSL"
                  },
                  "className": "text-right"
              }, {
                  "data": "RLMTime",
                  "className": "text-right"
              }, {
                  "data": "DropOfTime",
                  "className": "text-right"
              }, {
                  "data": "AccountName"
              }, {
                  "data": "EventCode"
              }, {
                  "data": "PassengerName"
              }, {
                  "data": "PickupCity"
              }, {
                  "data": "DropOffCity"
              }, {
                  "data": "ArrivalAirportCode"
              }, {
                  "data": "ArrivalAirline"
              }, {
                  "data": "ArrivalFlightNo"
              }, {
                  "data": "ArrivalFlightStatus"
              }, {
                  "data": "ArrivalFlightTerminal"
              }, {
                  "data": "ArrivalFlightGate"
              }, {
                  "data": "ArrivalBaggageClaim"
              }
          ],
          "order": [[16, 'asc'], [17, 'asc']]
          //"fnCreatedCell": function (nTd, sData, oData, iRow, iCol) {
          //    debugger;
          //    //console.log(sData, cData, 'TEST'); // not being done
          //    if (sData > 30) {
          //        $(nTd).css('color', 'blue')
          //    }
          //}
      });