Yes, golf balls or fake eggs work many times. Chickens tend to share nests, so where one hen lays her eggs, the others usually follow. Leaving something they perceive to be an egg will encourage them to lay in that same area.
On another note, if you can figure out what's so attractive about the floor and emulate that in the nest boxes, it might help. Do you use a different bedding on the ground? My hens can't get ENOUGH of straw bales. Nothing is more exciting to them. They abandon all nest boxes if there is a straw bale anywhere in the vicinity.