Usually, before you can see signs a hen is sick or even injured, the chickens will notice it and begin a campaign to drive the hen out of the flock.
Over a year ago, my five-year old EE Flo was suddenly getting brutally beat up by a lot of the flock. it became worse, and she was so stressed by the daily beatings, she quit laying all together. I believe she laid all of one egg all last year.
Come to find out, Flo was going lame. She was having trouble with her legs, hopping up steps, getting onto the roosting perch, and running away from bullies was out of the question. She took to sleeping on the floor under the nesting boxes, but another hen discovered her hidey-hole and co-opted it for herself to sleep in.
I finally had to remove Flo from the flock all together for her own safety. She sleeps in a wicker basket in the garage at night, and in the morning I carry her out to the run where she is safely incarcerated in the chicken jail. I rigged up a dog crate with a nest inside after she completed molt in hopes she might begin laying again. Surprise! She's been laying regularly ever since! She's still lame, and can't even stand on some very cold days, but she's no longer stressed out like before. But the minute she talks me into letting her out to mingle with the others, she immediately gets stomped. So the flock still knows she's lame and still wants her out of the flock.
There is something about your GLW hen that isn't right. Your flock knows it. You need to observe her carefully until you figure out what it is.