I've got a few ideas, but these articles and the thread may give you more clues.
Virginia Tech Stopped Laying
http://pubs.ext.vt.edu/2902/2902-1097/2902-1097.pdf
Florida Stopped Laying
http://edis.ifas.ufl.edu/pdffiles/ps/ps02900.pdf
Missing eggs thread
https://www.backyardchickens.com/forum/viewtopic.php?id=190121
I think since you are consistently getting one egg from one hen, the odds of a predator or a chicken eating them is not real high. It is still possible, but why would they eat all but one? I think that is a good sign.
Since they are free ranging, my first guess is that they are hiding them on you. How big is that fenced area they are roaming in? Any good hiding places?
You state they have never laid. Hens that age should lay quite a bit. How long is never? A few days, a few weeks, several months? Some hens can finish a molt in two or three months and some take five or six months. During a molt, they either shut down or cut way, way back in laying.
How sure are you of their age? Hens two years old are right about their prime in laying. Even at four years of age, you should be getting a few eggs each week from them. If they are six or seven years old, they do cut way, way back.
It is possible they have a disease and Betty is immune to that disease. The articles cover that.
If they are getting large amounts of cracked corn compared to the layer pellets, that will reduce the laying, but it will not stop it completely. You should still be getting some eggs from them. It has not stopped Betty from laying.
I strongly suspect that they are hiding them from you. Yuo might leave them locked up for a day or two to see if you get any eggs from them.
Good luck! This one can be easy or it can be real challenging.