Vista Tutorial - Issues using RenderTargetBitmap with a Viewport3D

I am attempting to save out an image of a Viewport3D using RenderTargetBitmap
but the image is not matching what I'm seeing in the Viewport3D.

The code is straightforward enough...

// Begin code
int width = (int)myViewport3D.RenderSize.Width;
int height = (int)myViewport3D.RenderSize.Height;

RenderTargetBitmap bmp = new RenderTargetBitmap(width, height, 96,
96,
PixelFormats.Pbgra32);

bmp.Render(myViewport3D);

JpegBitmapEncoder jpgEncoder = new JpegBitmapEncoder();
jpgEncoder.QualityLevel = 100;
jpgEncoder.Frames.Add(BitmapFrame.Create(bmp));

using (Stream stream = File.Create(@"C:\temp\output.jpg"))
{
jpgEncoder.Save(stream);
}
// End code

The jpg gets created, it's the right size and has some of the contents in my
viewport displayed in it, but not all. Some of the ModelVisual3D's are not
displayed, some are only partially displayed and some look great. It
definitely does not match what I see on the screen at the time the snapshot
is taken.

Has anyone seen this behavior before? Any thoughts/suggestions/questions?

Tom.