Egg reduction

Hi
Can anyone give me some advice on why 2 of my hens haven't laid for a week?
I have 4 hens and the Warren and Amber have stopped laying, they are 5-6 months old and are kept in an enclosed run so they can't be laying anywhere else. I have checked them for Mite and Lice and they were wormed finishing the course 2 weeks ago. They are eating their layers and drinking as normal, the vent area is clean and they aren't showing any signs of stress or illness.
Thank you

And I thought I was temperamental