http://www.gundogsonline.com/Article/Myths-and-Misconceptions-About-Retrievers-Page1.htm said:
The reference to natural retrievers is often used by the less knowledgeable people in the sport. For all practical purposes, there are no natural retrievers, some just come to you with a better starting point, i.e. better mouth, nose, water-going traits, bird interest, and so forth. In 25 years of professional training, I have had only two out of hundreds of retrievers that retrieved naturally -- and kept doing it. Many puppies will retrieve naturally for a short time but quickly realize other activities are more interesting. Those that retrieve well, naturally, into adolescence soon start dropping dummies, refusing to go, taking a deviated route back, etc. when the pressure to do things right is applied. Therefore, almost all dogs must be force-fetched -- a procedure that will take anywhere from ten days to a couple of months depending on the aptitude of the individual.