Hi Kathleen - You didn't mention a coop. Do they have a secure coop they can go into at night to roost? Are there appropriate nest boxes in the coop? They need a safe, dark, secluded place to lay their eggs.
With regard to the run (fenced pen), 1000 square feet is a pretty big area. I'd recommend thoroughly checking it for eggs, beginning with the area under the pine tree where you mentioned the hens go to hide. Thoroughly check around and under all vegetation and any other potential hiding places in the run. If they've started laying in the pen, you may need to confine them to the coop for several days to re-train them to lay in the nest boxes. If you see no signs of eggs after searching the pen, it may just be a case of them being late bloomers. They may start laying in a month or so due to increasing day length.
I'd also recommend raking the rock and stones away from a portion of the pen, so they have a place they can scratch around in. A 10 x 10 foot area should be fine to start out. After the stone is raked away, there should be exposed, natural soil. You could throw some scratch into that area to encourage them to scratch around in the soil. The only problem would be if this would create an erosion problem, so consider your local conditions (soil type, slope, erosion potential, etc.).
