FrameBuffer copy of scene only show sky and nothing else

i try to have another view of the main scene, copied into a framebuffer texture to use in the gui,

but the only thing that renders is the sky

Code:
lensCamera = new Camera(size, size); //cameraNode.getCamera().clone(); lensCamera.setAxes(cameraNode.getCamera().getRotation()); ViewPort offView = renderManager.createPreView("createSceneImage View", lensCamera); offView.setClearFlags(true, true, true); offView.setBackgroundColor(ColorRGBA.BlackNoAlpha);
        FrameBuffer offBuffer = new FrameBuffer(size, size, 1);

        lensTex = new Texture2D(size, size, Format.RGBA8);
        lensTex.setMinFilter(Texture.MinFilter.Trilinear);
        lensTex.setMagFilter(Texture.MagFilter.Bilinear);

        //setup framebuffer to use texture
        offBuffer.setDepthBuffer(Format.Depth);
        offBuffer.setColorTexture(lensTex);
        //set viewport to render to offscreen framebuffer
        offView.setOutputFrameBuffer(offBuffer);

// rootNode.updateGeometricState(); // not needed for rootNode but for some other cases
offView.attachScene(rootNode);


i use the same method for creating views about some spatials attaching it to the offView and it works
but not for the rootNode
:(

is there something special about this case that i miss?
what is the trick ?