keep playing fetch with the antlers. instead of "hiding" them, just set them in the open, but lock your dog inside or where he/she cant see where you are putting them. once the dog is bringing those back, then gradually start hiding them in more difficult spots (but where they are still visible like a tine or base sticking out)such as next to a bush or in tall grass, then eventually to where they are not visible. praise and reward can go a long way. the only time my dogs get treats is if they bring back an antler. once they get the treat, i throw it for them a time or two, because they LOVE to fetch. really over exagerate the "GOOD BOY!!" get on your hands and knees and pet him a lot....if he sees you so excited about him/her bringing you the horn, he will get just as excited!
Hope this helps, good luck, and keep us posted!!