Thanks for the replies. I had wondered about it being a worm burden, although she was wormed with the rest about 4 months ago. They are on layers pellets - I can't get waterfowl pellets nearby and they have all thrived on these for the past 7 years. They free-range normally (though not any longer obv) so supplement their food by foraging, and also have vitamin drops in their water occasionally.
She laid an egg this morning, bless her!
I will worm her, give her extra vitamins, and see how she goes.