have you stopped the pup chewing mouthing or biting? some dogs take it too far and then hesitate to use there mouths. i wouldnt worry about it as dog is still young, have you tried using a ball or teddy on a rope or piece of string? try getting the dog excited and chasing it around using praise when the dogs grabs it, only actualy throwing it for a retreive once the pup is using its mouth. dont give up, keep at it, itll come good or ill eat my shorts