A lot of performance is probably dog specific, some just have more natural instinct when it comes to tracking and searching out cripple ducks. I got lucky that my dog has a great nose and almost always finds crippled birds, she has found cripples left from previous hunters even! To help teach the skill you can do the "hunt'em up drill". Take the dogs bumper or even better a wing or intact duck and hide it in some brush, but don't let the dog see where you have hidden it. Next take the dog in the area where you know the mark is, point into the area and command "hunt'em up!" Pretty soon your dog will better learn how to seek out hidden birds. Also make sure the dog has a good line of sight in the blind, long retrieves when the dog didn't see the bird drop are quite difficult even for highly trained dogs.
