WPF Scrollviewer won't scroll up

I have the following (simplified) xaml:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="143*" />
        <ColumnDefinition Width="135*" />
    </Grid.ColumnDefinitions>
    <TabControl Margin="12,29" Name="tabControl1" Grid.ColumnSpan="2">
        <TabItem Header="TabItem1" Name="tabItem1">
            <ScrollViewer Height="440" Name="scrollViewer1" Width="872" HorizontalAlignment="Left" VerticalAlignment="Top">
                <Grid Height="440" Name="grid1" Width="851" VerticalAlignment="Top" HorizontalAlignment="Left">
...

My problem is the scroll won't show up. What should I chang开发者_高级运维e ?

Thanks.

---

The content inside the ScrollViewer is smaller than the ScrollViewer itself. The ScrollViewer is used for being able to use scrollbars to show content that is larger than the area the ScrollViewer takes up.