Ability can be trained to some extent, but the basics have to be inborn. The dog needs good eyesight and depth perception (to lock on the duck and see where it falls), a good memory, a good nose to sniff out the duck once he's in the area of the fall, and the ability to triangulate (figure out how a duck is drifting in the water with wind and current, and meet it at some point). Plus he has to have that elusive quality termed "style" or "drive" - he has to retrieve like that duck is the most important thing in the world and he can't wait to bring his prize to you.
Obedience is really important though, especially in the higher levels. The dog has to look where you tell him to to watch for the mark. And you can get away with a dog pulling to the line in Started, but in Seasoned he better be at heel and solid as a rock (no leashes or collars allowed after Started). And handling to a blind requires that the dog trust you absolutely -- my Chocolate in training still looks at me like, "Wait a sec. You are trying to tell me where a duck is? You with your pitiful nose? Who the heck is the bird dog here, anyhow?"
I think the AKC judges just make the tests too difficult for inexperienced dogs. Nothing in the tests I went to was unfair, exactly, it just required too much savvy on the part of the dog. Things like asking the dog to run to a mark uphill (slows him down) into a tree line (makes it hard to see the duck against the dark background) on the other side of a ditch (a natural boundary or 'stopper' for most dogs). An experienced dog knows he has to run harder uphill to travel the same distance, knows what a duck looks like against a dark background, and knows that a ditch or a fencerow is not a boundary. The worst mark, though, was a "live flyer" - that is, they had gunners in the field who actually shot a duck - that was shot over an overgrown mound of dirt that somebody had bulldozed up in the corner of a partially mown hayfield. Because you never know exactly where a live flyer is going to come down (the ducks that are already deceased and thrown out of catapults to the accompaniment of blank gunfire always land within around a six feet circle target area), the marks landed in top of, behind, beside, and in front of this 6 or 8 foot tall mound of weeds and dirt. So the dogs didn't get the same mark -- the ones who had their duck land BEHIND the mound were really at a disadvantage. Most young dogs don't know to go around something and have a look behind.
My dog got a 'gimme' on that one -- her mallard landed on the front face of the mound, in a clear patch, with the white breast feathers up and ruffled. Might as well have been a large neon sign flashing DUCK * DUCK * DUCK .
Even large neon probably wouldn’t help Buster. The best thing he has going is that nose of his. I bet your Chocolate is awesome! I love those but I get Weimeraners and Labs confused. Hubs and I were rodeo volunteers and saw lots of them both. Great hunters. Very pricey, as you said. About $1200 each when we left 5 years ago.