A lab will retrieve it is natural, now not coming back that is because you have always used a ball and dog park like you said.
I would suggest:
1. Not going to the dog park for awhile
2. Get a kid 50' check cord
3. Work on the retrieve and return now that you have something attached to her and can control her coming back
4. It will take some time but reward her each time she comes to you and gives you the bumper
