Chucks have stopped eating their feed

My hens have stopped eating their feed and I don’t know why. They had stopped eating their pellets and I had not changed their brand , so I moved to mixed corn which they ate well but since they have gone into moult they have stopped eating anything from their feeders, they wander round the garden and eat the grass but nothing else really even if I give them treats, meal worm they don’t really eat them but a few pecks . I have put Vitamins in their water to try and help.l am very worried, does anyone know why this could be or can help me? Is there enough in the grass for them? They are behaving out of sorts