I have an 8 month old heeler terrier x, he is an awesome dog, very smart and mostly does has he is told. We have been doing positive reinforcement as our training.
He still won't retrieve a ball, but will retrieve a basketball or football by pushing it with his head. If we take a tennis ball to the park he either isnt interested or becomes possive of it with other dogs.
If someone else plays ball with their dog he will do his best to steal it and this has resulted in some fights. We did manage to get him to give the ball for a treat for a while but he has stopped doing that now and definately won't do it if another dog is close by.
He also becomes posessive with sticks, grass, smells on grass, other toys at the park. At home with humans he shows now resource guarding, we can take food off him without any agression.
He really likes the chase, we have made sure we havent chased him with toys at home and have been ignoring him when he doesnt give up toys. Still at the park he loves the train.
He loves the park and I dont want him to stop going but I am scared he will get injured in a fight or injur another dog.