Let me start by saying I do not do Shc, but hahve to do a formal retreive for an exam.
Will your dog retreive other items to hand? Or is she a "spiter outer"?
I had a dog that would not hold and wanted to drop things at my feet and certainly did not want to sit and hold at the same time.
This is what worked for me. Firstly, I never picked another thing up of the floor for the dog again. Any casual retreiving required the dog to deliver to hand. Was frustrating in the begining, but I just kept moving back end encouraging the dog to bring it all the way to me and then the dog got that I was never going to pick the item up off the ground and throw it.
Next. I ditched my formal retreive item and worked with something else. I noticed he would hold a sock in his mouth and that he like to tug. I put a ball inside the sock. Worked on tug...I let go of tug, I move back, dog comes in for more tug, we play tug, I let go of tug, I move back, dog comes in for tugging and so on.
Once the dog was anticipating tugging, comming to me and holding the object, I casually asked him to sit. And he finally did with out dropping it. Butt touched the floor, item still in mouth, Woo hoo, mark reward by tugging.
Once we had finished working on the dog running to me and presenting for tugs, I put it into a formal retreive. Short at first. Dod sits at heel, item thrown, dog released, dog brings item back for tugs. I would move back and encourage verbally the dog to come right to me at first, but he quickly learned tugs were comming.
Once it the retreive was formalized, the dog knew what was expected whenever we started a retreive from a heel, the exersise would finish with the dog holding in front. I then started to change the retreive objects. First to a leather tug, for a few weeks, then to a jute roll and so on. Now that the retreive was fomalized and the dog knew what was expected, the item could be changed up. The out was rewarded by giving the item directly back to the dog.
Once the dog knows exactly what to it hased to do to get rewarded it will comply to get it's reward. I am not saying this meathod will work for you, just trying to illustrate that sometimes you have to get creative to get the dog to do what you want! I tried some more negative meathods, but my dog would shut down (he is soft). I made it a tug game until he knew what I wanted and then faded out the tugging. Sometimes I spit food with the returned item, with you it sounds as if it will be trading for the ball. Now that he knows the game, the retreive IS the reward.
You just have to figure out a way that the dog knows the reward comes only if the item is brought back and presented. For me that was making the reward item and retreive item the same thing in the beginging.