My chicken appears to be starving itself!

I have two Rhode Island Reds.
They have never been very keen on pellets from when I first got them. So i tried different brands. I resorted to giving them mash and putting the pellets in that, and I felt that they got enough as they are also allowed out in my small garden. I did give them scraps including some pasta/ rice sometimes. Recently however, they have stopped eating any chicken food at all. So I have stopped giving them any alternatives. Just pellets or mash. They are really hungry and rush to the door to get the smallest scrap of food. They will eat anything other than chicken food! I am getting worried as they seem so hungry and they are getting thin. I took one to the vet and he was none the wiser. We wormed them, but i ended up cooking up barley and lentils to put the medicine in!