THANK YOU SO MUCH JulesP. This is exactly what we are dealing with and I was starting to doubt whether I knew what I was doing (new dog after many years). Our GR who is about to turn one is exactly what you describe above: needing to unstick him, taught not to follow the treat, etc. I've started praising him when he shows interest in the treat and he is starting to recognise "Good Boy" may mean a treat is on offer.
Lord knows how the breeder plans to show him again occasionally after he's been living in a family home and learns traditional obedience!