How do you teach the dumbbell retreive? I have heard all the following
Some force open the mouth and place the dumbbell
Some back tie and put pressure on the prong collar (or other collar) until the dog opens his mouth. then place the dumbbell
Others use play drive to stimulate the dog into biting the dumbbell. What if the dog doesn't have the drive?
Still others pinch the ear
Don't do any without the help of an experienced trainer.