If your other one just started to lay, this one may be working on that first egg too.
Do a search for 'Egg and Bound' and see what comes up... She may be egg bound, or may just be having a hard time with the first egg.
Also, I agree - nutrition may be your main problem here. You need to reverse the ratio of scratch to layer pellets. Also make sure they have access to calcium source (like oyster shells)
At this point, a small amount of black oil sunflower seeds might help lubricate the egg machinery too.
If you decide to take her inside/isolate her, pick a draft-free spot. Try soaking some layer pellets in warm water and see if that tempts her. Some good poultry vitamin/suppliment might do her some good too, like AviaCharge. Follow the label directions on whatever you get.
Hope she turns around for ya soon.