SurfaceScrollViewer Pinch to zoom doesn't work

I have a SurfaceScrollviewer as below :

<s:SurfaceScrollViewer x:Name="ScrollView" HorizontalAlignment="Left" Height="Auto" IsManipulationEnabled="True" 
                               VerticalAlignment="Center" Width="Auto" 
                               PanningMode="HorizontalOnly"     VerticalScrollBarVisibility="Disabled" 
                               HorizontalScrollBarVisibility="Visible" 
                              ManipulationDelta="scrollview_ManipulationDelta"
                              ManipulationStarting="scrollview_ManipulationStarting"
                              ManipulationInertiaStarting="scrollview_ManipulationInertiaStarting">
             <StackPanel Name="stkPanel" HorizontalAlignment="Center" VerticalAlignment="Center" Orientation="Horizontal">

             </StackPanel>
</s:SurfaceScrollViewer>

and in the code behind i'm add some UserControls in to the StackPanel inside SurfacescrollViewer.

When i try to do  "pinch to zoom" gesture to zoom it just won't work!

if i try to pinch zoom the container of surfaceScrollviwer (Parent of the UserControl) zoom works and i can scale but when i try it from inside the surfacescrollviewer not!

any help would be appretiated