Sorry, not sure what to suggest, but hope she is ok. I have one who holds back from the bunfight, but she always has. She is very timid compared to the others, and if you approach her she is a bit like a dog, kind of rolls down waiting to be stroked or picked up!
It's incredibly cold, perhaps she was just a bit out of sorts and hopefully she will be keen for the treats today.
One of our girls was behaving a bit oddly this afternoon. They had been out in the garden for hours and I went out with a tin of sweetcorn to take them back to their run. Usually as soon as I come out of the back door they all come running to meet me, and today all 6 of them came. I threw them some of the sweetcorn and 5 of them did their usual pecking and ate all the kernels. But one of the girls just stood by and watched. Then I ran to the end of the garden like I usually do, and most of the girls came running with me, but this one hen stayed back down near the house. I shut the 5 in the run and went back for her but she just stood there not moving. I gave her a few kernels but she didn't take any notice. Then I started trying to chivvy her up the garden but she wouldn't really move unless I was pushing her so in the end I picked her up and took her back. I know it's not a big thing but it's the first time I've seen any of them not go for food, and not come running up the garden with me.