Xamarin.Forms: GroupHeaderTemplate is not visible, neither are the items but seems to partially render

I have a GroupHeaderTemplate in Xamarin.Forms for a ListView element which is not rendering. You can select the headers, and pull up the quick menu. I can successfully render the groups if I use the property of the ListView element GroupDisplayBinding, yet when I specify GroupHeaderTemplate property of ListView nothing renders. In both cases my ListView ItemTemplate does not render. If I remove the header, the ItemTemplate renders as expected.

<ResourceDictionary>
      <local:DateConverter x:Key="dateConverter" />
    </ResourceDictionary>
  </ContentPage.Resources>   
<ListView x:Name="barGraphListView" ItemsSource="{x:Static local:BarGraphModelColl.bgModelColl}"
              IsGroupingEnabled="True"
              HasUnevenRows="True" HorizontalOptions="FillAndExpand" VerticalOptions="EndAndExpand">
      <ListView.GroupHeaderTemplate>
        <DataTemplate>
          <ViewCell Height="25">
            <Label Text="{Binding Date, Converter={StaticResource dateConverter}}" TextColor="White" />
          </ViewCell>
        </DataTemplate>
      </ListView.GroupHeaderTemplate>
      <ListView.ItemTemplate>
        <DataTemplate>
            <StackLayout Orientation="Vertical" Padding="0,10">
              <Label Text="{Binding DataCategory}" />
              <BoxView WidthRequest="{Binding BarHeight}" HorizontalOptions="Start" Color="Blue" HeightRequest="20" />
            </StackLayout>
        </DataTemplate>
      </ListView.ItemTemplate>
    </ListView>

This is my converter

class DateConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo cultureInfo)
    {
        DateTime dateTime = (DateTime)value;
        return String.Format("{0:dddd, MMMM d, yyyy}", dateTime);
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo cultureInfo)
    {
        throw new NotImplementedException();
    }
}