WPF Toolkit DataGrid control won't sort all my columns.

Question

I am using the WPF Toolkit DataGrid control and it works fine, with a minor exception. The last four columns of my table will not sort. All the other columns sort, and a up/down arrow appears.

I used my Server Explorer to drag and drop a table onto the Designer. My C# code looks like:

private void SymbolsControl_Loaded(object sender, RoutedEventArgs e) {
  using (SymbolsGroupDataContext dc = new SymbolsGroupDataContext()) {
    myDataGrid.ItemsSource = dc.SymbolsGroups.ToList();
  }
}

That's it.

My Xml lools like:

<dg:DataGrid x:Name="myDataGrid" 
             Margin="50,174,74,23"
             CanUserSortColumns="True"
             AutoGenerateColumns="True" />

When I click on one of the last four column, I don't see any UI change. No arrow. Am I missing something in my Xml?

It looks and feels like only the first four columns receive a click event, and the last four columns do not. It that possible?

Charles