Do you have a clicker? I dont remember you saying that you do/dont, so I'll just say what I would do with Bella.
What I would do, using a clicker, is throw it, and then call her. When she looks at you with it in her mouth, click and treat. When she moves towards you with it in her mouth, click, treat. Gradually moving closer gets clicks. Whe she comes to you all the way with out dropping it, jackpot.