A force fetched dog is one which has been successfully trained to properly retrieve, on command, in any situation at any time. There are many dogs that have a retrieving instinct, but unless successfully force fetched, you run the risk that at some point the dog will say, "Nah. I don't feel like it today." This behavior can manifest itself in many ways ranging from outright refusal to retrieve to mouthing the bird as the original poster is seeing and many others. Successfully force fetching the dog removes this variable. The term "force" is to be used loosely as different dogs require different means of stimulation to achieve the desired results.