We have no end of problem with Ollie and tennis balls, whereas Maisie is very good and will retrieve and give back. It means now she has to have separate ball playing time, otherwise Ollie will retrieve it and start shredding/eating the fuzz off the ball. If you go anywhere near him he growls, clamps it in his jaws, and runs off. It's so embarrassing if he picks up someone else's ball when we're out because he won't come to you and has to be grabbed and ball prized from his jaws. Not something we want to do, but there is no way he's prepared to give it back. We've tried the high value treat swap, but nothing is as high value to him. Ignoring and walking away seems to reinforce "its ok, i'm allowed to have this" in Ollie's case - you can almost see a look of smug satisfaction! We've now taken to keeping a spare ball with us to give to the other people in the event Ollie steals theirs and if we spot someone ball throwing in time he gets put back on the lead. I only hope Maisie doesn't pick up his bad habit and ball obsession.