ScrollViewer enveloping a Grid not working properly - wpf

Question

I am facing a strange problem with the ScrollViewer
I have a ScrollViewer with a Grid inside. The height of the Grid requires a vertical scrollbar. At runtime, the scrollbar is visible, but appears disabled and has no scroll indicator between the two arrows.
However, just above this scrollviewer is another set of controls enveloped by a scroll viewer, actually just the wayas the above, but here the scroll viewer is working just fine
here's the code
<Grid><Canvas Height="250" Width ="400" Margin="0,0,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Background="White" Name="ModeCanvas">
<Rectangle Width="{Binding ElementName=ModeCanvas,Path=ActualWidth}" Height="{Binding ElementName=ModeCanvas,Path=ActualHeight}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Stroke="Black" StrokeThickness="2"/>
<Grid Height="244" Width ="394" Margin="3,3,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Name="ModeParent">
<ScrollViewer VerticalScrollBarVisibility="Visible" >
<Grid VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Name="ModeGrid" >
</Grid>
</ScrollViewer>
</Grid>
</Grid>
</Canvas>
<Canvas Height="250" Width="400" Margin="0,255,0,0" VerticalAlignment="Top" HorizontalAlignment="Left" Background="White" Name="InputDataCanvas">
<Rectangle Width="{Binding ElementName=InputDataCanvas,Path=ActualWidth}" Height="{Binding ElementName=InputDataCanvas,Path=ActualHeight}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Stroke="Black" StrokeThickness="2"/>
<Grid Margin="5,5,5,5" Width="390" Height="240" Name="InputDataParent">
<ScrollViewer VerticalScrollBarVisibility="Visible" >
<Grid VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Name="InputDataGrid" >
</Grid>
</ScrollViewer>
</Grid>
</Canvas>
<Canvas Margin="0,510,0,0" Height="250" Width ="400" VerticalAlignment="Top" HorizontalAlignment="Left" Background="White" Name="OutputDataCanvas">
<Rectangle Width="{Binding ElementName=OutputDataCanvas,Path=ActualWidth}" Height="{Binding ElementName=OutputDataCanvas,Path=ActualHeight}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Stroke="Black" StrokeThickness="2" />
<Grid Margin="5,5,5,5" Width="390" Height="240" Name="OutputDataParent">
<ScrollViewer VerticalScrollBarVisibility="Visible">
<Grid VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Name="OutputDataGrid">
</Grid>
</ScrollViewer>
</Grid>
</Canvas>
In this code - the first 2 Canvas render the scrollbar just fine, but for some reason the third one is creating a problem
Any Ideas ??
EDIT
Finally got the mistake in the code. The probelm was in the code behind.
The basic probelm was that when we resize the window at that time some code behind code was changing the heights of the first 2 Grids but not of the third Grid. Initially the data that was inserted in each Grid(dynamically) was such that, the first 2 Grids used to overflow but not the 3rd one, . So, now when i resized the window the third Grid still did not have a scroll viewer even though now the data was overflowing the space,giving an impression that the scroll viewer was not activating.
However, i missed the simple fact, that the height of the 3rd grid was not changing, whereas the first 2 Grids height was changing, thus the scroll viewer for the first 2 grids was properly working but not the 3rd.

Answer 1

The Grid inside the ScrollViewer is empty. What the scrollviewer is expect to scroll?