FWIW this works for me espcially with a toy like the one above. When the dog returns the but refuses to release the toy I train "give" this way. I will crab the toy with one hand on each side and close to the muzzle. Holding the toy very still. Not tugging when the dog tugs just hold the toy still. Dog will lose interest in "dead toy" and release it. Wait until they fully release. Many a dog will try and play of game loosening the grip on the toy but as soon as you try and take it from them the grab and pull back. At first it will take some time each time the dog return but if you are consistent the dog quickly learn the only way the toy becomes alive again is when he releases it. When the dog is consistently relasing the toy you can then start to add a command. Just before the dog is going to release the toy say the command "give" For instance" this word sonn will become a predictior for the behavior. It works much better to train the cue after you have the behavior. Remember dogs do not understand language as we do IF you start telling the do give give as soon as he return with the toy to give, he has no idea what that means. If in responce to him not giving up the toy you try and pull it away from him and do this consitently over time you have trained the dog a very important meaning to the word give. Give becomes you command to play tug of war. An the dog will learn this very quickly. Always get the desired behavior before train the cue.i just wish she could work out that she needs to drop the toy for me to be able the throw it again LOL - silly dog!
If the dog is not reliable in returning to you with the toy, you can try and tie/sew something on the end so you can pull the dog in. Dogs quickly learn the fun of any keep away game. By teathering the toy a Playing keep away is no longer an option so you can the start training the desired retrieve behavior.
Encourage Natural Retrieving In A Puppy