I used to have the same problem with my dog at fetch. She'd get it then just run around with it keeping it from me. This is the method I used to get him to bring it to me:

1. Buy a whistle

2. Start blowing the whistle at random times of the day and give your dog a treat every time

3. Eventually your dog will come running to you as soon as he hears the whistle

4. Bring the whistle and treats outside while you play fetch

5.After your dog fetches the ball blow the whistle