A retrieve isn't chasing the ball, it's putting it in your hand. Focus on the delivery!
"He'll get it, but he won't give it back!"
This is not a problem of refusal. It's not a power play. The dog has simply not been taught how to deliver the ball. This isn't a dog problem, it's an incomplete teaching problem.
Dogs already know how to chase things.
Dogs need to be taught how to deliver to hand.
Ellie has returned with the ball and placed it carefully in the hand.
Teach a willing exchange, a clean and consistent delivery, and you'll have a reliable retrieve!
A retrieve is a back-chained behavior.
The next throw rewards the return and placement in your hand. You can start by exchanging for a treat to teach the dog to target the hand. *click* or "Yes!" The dog will release the ball to take the treat. When this is strong and reliable, fade the treat - the next throw will become the reward.