It's just a suggestion but I wonder if a return to basics might help. Is it that she gets tired, bored or frustrated. There are two things that I would try and I'm not sure if either would work. Firstly. as daft as it sounds just get her to retrieve from an arms distance and reward her with however she responds best. after a couple of days try a leg length, then a body length. You get the idea. Another way could be as she is on her way back turn and walk away, she might keep coming without dropping short. I was very lucky with mine I never had that problem but I did have plenty of others. I'