Chickens with lice help!!?

I picked some hens up recently and have discovered they are riddled with lice! Ive been treating them with louse powder and red mite powder but am getting nowhere! WHat am I doing wrong? Also would the lice stop them laying? (they only have the yellow lice not red mite just using that powder as a preventative) Is there an easier way to get rid of the lice than the powder?? Im in the uk and have 5 hens. Many thanks