Apologies for shouting but my endless patience is starting to wear. 
Adult dog came to us relatively untrained at 18 months, picked up the basics quite well but still struggling with the leave.
He's fixated on balls and loves to retrieve, the only problem is getting the bloody ball off him after he's come back! 
He's really good at respecting your 'space', if you like, so if he accidentally drops it and it's nearer to you, he'll leave it alone and let you pick it up. He'll be interested in the ball if it's in your hand, but he doesn't try to 'mug' you for it and will run ahead some way but keeps coming back to check you have the ball still.
Ideally, I'd like to train him to retrieve, sit or lie and leave it as he's a big dog and whoever is walking needs to be confident he's not going for the ball when you go to pick it up.
Right now, the retrieve is fine, I just can't understand why he won't give it up if he loves chasing it so much..it's like the possession of it is more important.
I'm being calm, firm, consistent etc and he's well trained otherwise and behaviour has improved 1000%, just struggling a bit with this.
Any suggestions would be most welcome.