It kind of sounds like the dog is getting bored of the "game". As soon as the dog is no longer engaged with retrieving you have done too many throws, rather you want to keep the dog wanting more. I would say limit the number of throws the pup sees, such that the retrieve is the reward. If the pup loses focus after 3 throws, start out with only 2 a day. Keep the sessions short and allowing the pup to play with the dummy rather than simply taking it right away.
The trick is keeping the dog's attention, if that takes feathers, use feathers for now. After sometime of limited exposure, then gradually increase the number of retrieves the dog sees If the dog loses focus, take a couple steps back. Once you've increased a couple of throws, lose the feathers and drop back down the number of retrieves. Once again, if the dog loses focus, take a couple steps back.
T