Hi,
I agree entirely with Miche - its excellent advice. I have recently read the Victoria Stillwell - Its me or the Dog book and having grown up with dogs, was surprised how little I knew about behaviour and responses. Although I consider her slightly patronising, I cannot deny that she knows her field.
Her advice is to teach comands in a positive fashion - if you haven't taught your dog to leave, he cannot be expected to understand this concept himself. "Leave" can be taught by using a treat in a closed hand and rewarding with a treat in the other (un-noticed by the dog). Building up to leaving a dog alone with the object of desire.
I don't mean to preach, as my 4 year old, Clunie, will never return a 'retrieved' ball, but only hide it from me, but I've decided to work on it for my own sake and that of my poor shoes, as she only hides the balls in the swampiest, nastiest places!!! Thank God for £0.88 for 3 balls at Tesco!
Best wishes and I hope this is sorted soon.
Love
Gill and Clunie