Hen losing weight

Have had Dora, a ginger ranger, for two months and she has lost weight compared to my other hen, a Pied called Kate, which i got at the same time. Was told to worm them so have used Vermex for two consecutive months. Dora seems well enough in herself, eats and poos ok, lays an egg a day, but her breast bone really protrudes now and she looks considerably thinner than Kate. Any suggestions?