Hi mate.
When you play with her will she take her toys back to somewhere she feels safe,ie,her bed.If so,try sitting in her bed & then playing fom there.When she brings it back to her safe place, you will be there.Keep doing this until she gets the idea.Lots of praise when she gets it right.Just keep it fun & don't over do it.As has already been said.Not all dogs are natural retrievers & take more work & stronger bond & trust.Personally i think you are expecting a lot at 15 weeks.
Good luck.
Atb,Kev.