ksnecktieman gave you some great advice. Thinking like the dog will help. One thing I've done to get the message with dogs who display this confusion: Starting from the point that retrievers all enjoy retrieving...it's either inherently "play" for them, or the reward they get makes the whole thing worth it to them. "What's in it for me?" is a great statement! They decide only "good for me/not good for me" or "fun/not fun."
A very basic question. Will your dog play? Will he tug on a rubber toy when it's in your hand? Something tells me this might be a good place to start. If they find out that you get involved with them and the toy, you're already starting to win. It's a two edged sword, though, a retriever should respond to an "out" command, and let go, but that's a later battle. Just find out if he "gets" the idea of playing. Yep, some dogs have to learn how to play!
If you get him interested in playing with objects, all you then have to do is toss it just a little distance...not even arm's length. If he picks it up, expecting to play, do it for just a second, then "trade" him a piece of hot dog, ala ksnecktieman's advice, using whatever your normal command for release might be, and you're off and running. Remember to never make a game of chasing him if/when he begins to show interest in toys...that's a hell you don't want to start!
Things I've seen cause this? Dogs who have things tossed at them to stop some negative behavior [those dumb-a** "boogie cans], or have had things thrown at them making contact. Dogs who started out life/puppyhood chasing anything that would interest them, and got yelled at or punished for it. Dogs who've been put on long leads, started running around and playing, and had the "death jerk" put on them when they got too far. [Some people use this as a training device for keeping dogs in the yard, but it backfires when a dog won't run "outbound."] Dogs who have chased things and hurt themselves...will remember that. And, in agreement with the others, some puppies just have to see the "real thing" to get it, or just grow up a little more.
Positive and relaxed is the way to go. Set the dog up to succeed, make small steps, keep your expectations very, very tiny. Reward only when you get what you want, but keep what you want small and doable by the puppy. It it's not working, back up to rewarding him for anything you can get him to do, thereby ending on a successful note.
"Making" him do anything with any other method than positive reinforcement may work, I suppose, but it will cause all sorts of problems later, in a whole array of areas you never intended.
*Late note* Another thing I've run into is a puppy who was "trained" by an older, more experienced retriever that he would never be able to get to the fetch object first. In other words, the puppy thinks, "why should I go after whatever that thing is? The "other dog" always gets to it first!" They give up when they have no chance. But, as some have suggested above, it is also possible that seeing another dog retrieve might put two and two together for him. Dogs are naturally competitive when it comes to toys and food and affection, sometimes it'll work with you, sometimes against you.
