5 1/2 mo. old Golden that won't retrieve

I got my puppy at about 3 months old from a respected breeder who said they used their Goldens for pheasant hunting. I started training him to retrieve a few weeks later, hoping to take him hunting with me some day. He did great for the first month or so, but then started refusing to retrieve. He'll run after the ball, pick it up, and then drop it and do something else. This coincided with the dropping of his second testacle - a dominance problem? I don't know what to do to get him back on track.