Can you feel an egg inside her? Is she straining or doing a walking squat, staying in the nesting box for long periods with no egg. If you have one you think is egg bound give them a warm bath let them soak for maybe 30min at most. It might take a few baths sometimes it works immediately.
My girls usually start messing up the nesting boxes almost like their digging a hole. I also see them resting in them a couple weeks before they start laying.
