There is not much you can do. They will lay when they are ready, and that's all there is to it.Anything I can do to encourage the other hens to start producing?
The exception to that: longer days DO make a difference. So you probably could stimulate them to lay by providing artificial light to make the days seem longer (it often takes a few weeks to show an effect-- they might be laying by then anyway.)
There is the question of whether artificial light is bad for them, if it causes them to lay eggs sooner than they otherwise would. (But the same question would apply to naturally-lengthening days in the spring, for pullets that are maturing at that time of year.)