Hens stopped laying

All four of my ex bat hens which we've had for a year have stopped laying for the last week. We did have a problem with mites a few weeks ago but appear to have cured that.
They free range all day and they are fed layers pellets. They get about two cups of corn between them per day. I can't see any signs of them laying elsewhere and up until now have always layed in the nesting box.
Anyone got any suggestions?

Brian