QVTKOpenGLNativeWidget and volume renderering

I have a Qt 5.13 app with 2 views and a splitter. I’m using QVTKOpenGLNativeWidget.

I can render geometry no problem - cylinders, etc.

But I cannot render a vtkVolume.

Anyone have any sample code that works? I can strip mine down a bit and share it if someone will look at it.