That is normal with chickens. They all like to use the same one nestbox. In big flocks, there might be several favorite boxes, but they still line up and argue rather than using the other ones that are sitting empty.
It's good that you have more than one nestbox (in case some hen gets desperate and can't wait), and the golf balls are a good idea too, but your chickens will probably just keep sharing the favorite one. There's not much you can do about it.
You can check whether there is actually anything wrong with any of the others (water leaking, or bedding all scratched out, or something like that.) But they are probably fine, and your chickens are just doing what chickens like to do.