Bob? Might just try what I think of as "kitchen training."
I confine the dog, with short leash attached, to kitchen and take out some fetchable thing...toy, old winter sock tied in knots, etc. I sit on the floor.
Then we play fetch, but not tug-of-war. Lavish praise, congratulations each and every time. Inevitably, the pup tired of it and doesn't follow through, and then I just get a hold of the leash and tug it back, take the sock and tease the pup briefly and toss it out. Wash rinse and repeat. Great praise.
Throughout the day, I do the same with other stuff, stocking cap I might be wearing, glove, dropped pen--whatever. Same deal lotta praise, minimal criticism.
The sessions are short and always fun for the pup. Later, I introduce a pheasant wing and go through the same routine. With Mick, added a frozen quail because he seemed to be mouthing and almost chewing on a pheasant wing. In this instance, I just raised my voice and firmly said, "NO."
Then we'd continue. That poor quail got refrozen many times, but served valiantly.
Worked out, although throughout most of his 11 years with me, Mick would make fantastic retrieves, and then put the bird down 6-15 feet away from me, and then come back to me. Usually took about 3 encouragements for him to (grudgingly) bring the bird all the way. I didn't make a fuss, just accepted his token reluctance as part of our ritual. Then again, I mostly hunt alone.
Others here are wiser and more professional than I, but kitchen training works.
Be safe.