Blimey - this brings back memories! Paddy learnt everything but "fetch" was a totally alien concept to him - ran, caught the toy/ball/whatever, came back part way, lay down and chewed it. Would come back to me - but minus the toy.
It was mind-numbing but here's what I was told to do - slow, boring but it has worked, although he still pauses before brining things back to think about where his best interests lie.
1. Chase: a lot of advice given is never, ever play chase with your dog otherwise you'll b***** up the recall and fetch mechanisms. Good advice if you've never started. Only I'd already started playing chase in the living room with Paddy by the time I got this advice. Then I found advice from Jean Donaldson in "The Culture Clash" which said that you can play chase but with certain rules - one toy only, which is only ever used for chase games (i.e. you chasing dog) - and the dog has to sit/down/stay whenever you tell it to, otherwise you stop the game. You also use one phrase/tone of voice ONLY when chasing. Mine is (spoken like a simpleton, frankly) "Gimme, gimme, gimme") - and the toy is a soft furry ball thing. Any other time or toy that Pads runs away with a toy - I walk in opposite direction. Treat him whenever (mid official chase game) he obeys the sit and drop commands and really high-pitched squeals of "well done, good boy". (Told you I sounded like a simp).
2. Fetch: my real problem with Pads was/is that he is so food motivated, when I had treats to reward him for the right response, it was beyond his little lemon brain's understanding that he would move an inch further from the food he wanted. So "sit", "Down" etc - meant him staying where he was, near the food. Even "stay" and "wait" meant him being still and the food moving away. But "fetch" means him voluntarily leaving the food behind. No way, Jose or Hose B. So: excruciatingly slow - using clicker and treat, hold the toy you want him to fetch. When his nose touches the toy - click & treat. Do this several times, then put the toy down on the ground a few inches from you. When he makes ANY move towards the toy with his head - click and treat. timing is really important and I got it wrong (too slow or anticipating his moves) quite a bit to start with. After a few goes - he has to do more, so you want him to mouth the toy. Click and treat. Repeat, rewarding/reinforcing the right responses, completely ignoring (look over his head, look away etc) any wrong/blank responses. Then wait to click and treat till he gets his mouth around the toy and picks it up. Click & treat. Repeat. Then throw the toy a foot further away. Click and treat for any active, clear movement towards the toy. Build up the response (mouthing the toy, then picking it up - even for a second - in his mouth) before you'll click and treat. To get to this took me about two weeks of doing it for five minutes per session twice a night. It just extends further. When he's picking up/carrying the toy from further away, he knows you're still holding the goodies. If he drops it and trots back for a reward when you've got beyond that stage - ignore him, or just end the game and ask him to sit or do something easy, click and treat then end the game. When he's taking just a few steps back in your direction at any stage of this - give him the "BINGO" reward - several "high taste" treats. That's when I introduced the command "fetch".
After about three weeks, I saw real improvements in Paddy but there were two "step change" events that happened. The first was that the petsitter started playing fetch with him, and her two dogs were already trained to it, so he had their example - and competition for the toy and the treat, so that made a massive improvement in a matter of days. Second one was getting one of those "chuck-it" toys for flinging tennis balls a long way. Paddy thinks this is the best fun in the world, but almost instantly cottoned on to the fact that I can't chuck it if I don't have it to chuck. So he brings it back, but I still have to swap a treat for the ball to be dropped. he's never going to happily hand over "his" toy - yet he's not otherwise a resource guarder and I can ask him to drop anything (including the beer bottle cap he got hold of on Sunday) and he'll do it without the treat. But if the word "fetch" has been uttered: you'd better pay up, now, lady!
Hope that helps. Sadly - no quick cure!
Good luck and let us know how it goes.
Denise