Label showing in Preview, but not on iPhone

Hello,

I am walking through the ThingWorx Studio Tutorial and am invoking functions from a button.  When I perform the exercise and Preview, the label is shown and rotates as expected.  However, when I scan the target on my iPhone and select the Experience, the button shows, but the label does not.

What I do get is a half-toned ThingMark and the text "Point camera at ThingMark" underneath.  I point the camera at the ThingMark that triggered the Experience, but nothing happens.

I have placed the 3D Label above the plane, so it should always be visible, but it is never shown.

Note - I did upgrade my iPhone to iOS 10.3.1 today.

Any idea what I could be doing wrong?

Many thanks

Edward