
Some breeds and some individual dogs just don't do the toy retrieve thing, it doesn't float their boat.
My own bred is like that so you need to associate the retrieve with something that does float her boat, food!!!!
I believe it is called a forced retrieve, not is using force, but because it isn't one from a natural inclination.
As far as I remember you start with tiny steps, with the dog sat in front, and either encourage the dog to take the dumbbell or retrieve article, or put it in their mouth, and immediately give a reward saying hold. With the aid of treats you should be able to get her happy to hold the item for increasing lengths of time and a good enough reward.
Next step command hold when the dumbbell is an inch from her mouth and when she retrieves it that inch, same as before gradually increasing the distance, then lowering it, putting on the ground and increasing distance etc etc.