Eggs without yolks

[Hanako]

Two weeks ago one of my females laid 4 eggs, that appear to be without yolks. When I candle them, there is nothing in them at all. It is not one off, they all appear to be like that and they also feel a bit lighter for their size. Is she missing something in her food or why isn't she creating yolk? (I thought even infertile eggs would usually have yolks, wouldn't they?)