WPF DataGrid scroll bars not visible when HorizontalScrollBarVisibility and VerticalScrollBarVisibility set to Auto

I'm adding a DataGrid to a Grid and setting both the HorizontalScrollBarVisibility and VerticalScrollBarVisibility to Auto. When I populate the DataGrid, there are no scroll bars. ANy idea what I'm doing wrong?

var dgOrderData = new DataGrid { Name = "OasisOrdersData", HorizontalScrollBarVisibility = ScrollBarVisibility.Auto, VerticalScrollBarVisibility = ScrollBarVisibility.Auto};
dgOrderData.SetValue(Grid.RowProperty, 2);
dgOrderData.SetValue(Grid.ColumnProperty, 0);
dgOrderData.SetValue(Grid.ColumnSpanProperty, 3);
dgOrderData.ItemsSource = GetPlannedOrders((string)comboBox1.SelectedItem);
oasisContentControl.Children.Add(dgOrderData);