Viewport layer sorting problem

I’m having a problem sorting some viewport layers.  Two of the layers in the code below, menuPlaneLayer and gameBoardLayer sort fine, but the one that contains the Collada file (gameComponentsLayer) does not – it appears behind everything.  What am I doing wrong?

var testColladaMaterialsList:MaterialsList = new MaterialsList();

testColladaMaterialsList.addMaterial ( new ColorMaterial ( 0xD3C8AD), "ERDefaultMaterial"); 

var testCollada:Collada = new Collada("assets/componentmodels/testmodel1/testmodel1.dae", testColladaMaterialsList);

testCollada.rotationX = -90;

testCollada.z = -100;

_gameComponents.addChild(testCollada);

//Create gameboard plane

var planeMaterial:ColorMaterial = new ColorMaterial(0x);

planeMaterial.doubleSided = true;

_gameboard = new Plane(planeMaterial, 500, 400, 3, 3);

_gameboard.addChild(_gameComponents);

_gameboard.rotationX = 70;

//separate and sort layers for proper clipping:

var menuPlaneLayer:ViewportLayer = _viewport.getChildLayer(_menuPlane);

var gameboardLayer:ViewportLayer = _viewport.getChildLayer(_gameboard);

var gameComponentsLayer:ViewportLayer = _viewport.getChildLayer(_gameComponents);

menuPlaneLayer.layerIndex = 1;

gameboardLayer.layerIndex = 2;

gameComponentsLayer.layerIndex = 3;

_viewport.containerSprite.sortMode = ViewportLayerSortMode.INDEX_SORT;

_scene.addChild(_gameboard);

Thanks for any advice,