Video playback "falls through" the tracker

Let me start by saying I do not know OpenGL ES very well so any code in my iOS app (Objective-C++) to perform the rendering has been left as it was from the advanced samples video playback code.

I am experiencing strange behaviour when tracking an image and playing back video overlaid on the tracker; the playback works perfectly fine when the tracker is held away from the camera, but when coming closer, the video disappears as if it has fallen through the tracker.  I suspect this is impossible as the video is not a 3D space although it depends if the Open GL rendering creates a surfuce onto which it renders and the Z axis is set too low?

Anyway, I can see from the console the tracker is being tracked and can even still hear the audio from the video so the playback is continuing even the the video is not visible.

Could someone please point me in the right direction as to where I may be able to fix this?  I was hoping there would be a Z value to translate the video "up" a bit but that is clearly wishful thinking.

Any pointers or tips would be much appreciated - Thank you in advance!