golf balls didn't work for me - I had to switch to wooden eggs.
A couple of points -
If they continue laying on the ground & get stomped on, you could quickly get egg eaters on your hands and trust me - you DO NOT want that to happen!
I had a stubborn gal that refused to lay in the nest box. So, I worked with her...I used a rubber feed pan (like the ones you use for horses - low sides) and filled it with shavings and a wood egg. I placed it in the exact spot she laid her egg the day before. Took a couple days, but before long, she was using the feed pan. Then, when she was consistently laying in the pan, I ever so slowly moved it closer to the coop each day. If she laid back on the ground, I moved it back...eventually, after a couple of weeks, the pan was just inside the coop door, and from then on, she laid in the nest box.
Good luck!