Yup my 6 mo. old is that way, always has been. She will play fetch in the house forever. Out side it is 3-4 throws and it's over. But her flirt pole is another story. She won't stop playing that until she drops. Your pup is young and it just might be getting physically tired if you are throwing the ball far. Or, like in my case, she just isn't satisfied with the reward of fetch. If she is very prey driven a flirt pole may be more satisfying to her. Just my thoughts.
Ziva GSD 3/7/13
At the Bridge: