Why doesn't two-way binding of a Slider control work in a Windows Store app?

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C#

XAML

WinRT

I am trying to perform some simple binding in a Windows Store app. Some controls e.g. ToggleButton bind fine, but when a Slider is bound, changes in the UI are not reflected in the bound object.

The data source is an ObservableCollection<Film>. Film has some properties including:

public bool Seen { get; set; } // Actually INotifyPropertyChanged is implemented
public int Rating { get; set; } // on both.

The XAML is like (simplified for clarity):

<Page DataContext="{Binding DefaultViewModel, RelativeSource={RelativeSource Self}}">
    <FlipView DataContext="{Binding Group}" ItemsSource="{Binding Source={StaticResource itemsViewSource}}">
        <FlipView.ItemTemplate>
            <DataTemplate>
                <StackPanel>
                    <ToggleButton Content="Seen It" IsChecked="{Binding Seen, Mode=TwoWay}"> 
                    <Slider Value="{Binding Rating, Mode=TwoWay}" IsEnabled="{Binding Seen}" Etc="the usual properties set here.">
                </StackPanel>
            </DataTemplate>
        </FlipView.ItemTemplate>
    </FlipView>
</Page>

When the user interacts with the ToggleButton, the Film's Seen property is updated. The Rating property never changes. I have to manually updated the business object in code behind.

I have been googling all day. I couldn't find other examples of binding Slider.Value. Everyone is interested in binding it to the value of a TextBox.