Luckily I've found the secret. Birds. That's right. The secret is birds. Rudy gets bored with bumpers and balls, but his desire for birds never ends.
At a park near our house the doves feed on the ground everyday so Rudy and I often do our training there. Here's a video of him chasing a bunch up and retrieving for me. Sorry you can't see the doves very well. There was about 15 of them.
I throw doves which I shot last month (now frozen) instead of bumpers or his tennis ball and it has made all the difference. He's been able to learn how to do blind retrieves, in which he takes direction from my pointing hand because he didn't see where the bird fell, as seen here:
Often when sent on a blind retrieve his direction needs to be corrected, so I've tried to teach him to take direction from me. Because I use birds he cares enough to learn that when I blow the whistle he must sit, look at me and go left, right, farther, or closer depending on my what he sees my arms do. He has picked it up very quickly:
I've also been able to teach him that he needs to finish one retrieve before he can go get the next. That way if I shoot a bird while he is on his way back he won't drop the one he has for the one just hit. You can see here that he looks back and marks the fall, but then continues his fetch.
And finally a combination of direction and distraction.
What a good little dog. He'll never win any awards for speed. In fact he still thinks this is all just for fun, (which I guess it is). But even if he never fully gets it, I'm impressed. He is, after all, an unregistered pet-store pup taken from his litter at three weeks of age and considered not worth the effort to doc his tail to meet breed standards. Not exactly from a royal lineage. There might even be a little mutt in him, but like I said...I'm impressed.