Three of my dogs will fetch balls

Merlin, Pixie and Bess come hurtling back to me with their balls - time and time again. (This had to be a composite - no way can I throw 3 balls at once ! )
But what about Mystic? see extra - she will run after it or take it from Merlin, but bring it back? Not a chance - she stands there daring me to go to her, then picks it up again and drops it somewhere else!