Arcgis Runtime .NET for WPF scene with elevation source shows flat instead

I tried the Arcgis tutorial from here. It shows how to add basemap and elevation so that the scene would show a 3D surrounding, but it doesn’t. Can someone help me?

This is my .cs file

    private void Initialize()
    {
        // Create a new scene
        var myScene = new Scene(Basemap.CreateTopographic());
        Map3D.Scene = myScene;


        // create an elevation source
        var elevationSource = new ArcGISTiledElevationSource(new System.Uri("http://elevation3d.arcgis.com/arcgis/rest/services/WorldElevation3D/Terrain3D/ImageServer"));
        // create a surface and add the elevation surface
        var sceneSurface = new Surface();
        sceneSurface.ElevationSources.Add(elevationSource);

        // apply the surface to the scene
        Map3D.Scene.BaseSurface = sceneSurface;

        // Create camera with an initial camera position (Mount Everest in the Alps mountains)
        var snowdonCamera = new Camera(53.06, -4.04, 1289, 295, 71, 0);

        // Set the scene view's camera position
        Map3D.SetViewpointCameraAsync(snowdonCamera);
    }

And this is my xaml file

<Grid>
    <esri:SceneView x:Name="Map3D"/>
</Grid>