Ok,This will be longwinded and I am sorry but I want to get everything needed in...I have a just turning 2 year old Production Red from the feed store. She and her flock mate P.R. laid like crazy the first year (6 eggs a week minimum). Then over the winter she and her other flock mates (2 B.O.'s of the exact same age) did the normal stop of laying to moult and such. The hen in question went through the hardest moult (she was nearly bald).
Just as everyone was starting to lay a couple a week she laid a soft shelled egg - I didn't expect her to lay any since everyone else was done with their moult and she was still a bare mess. Then she laid a couple yolks with no shell. Then she got peritonitis.
I treated her for 12 days with a broad spectrum antibiotic and nursed her back to health. She was skin and bones by the time we were done since some days we could only get her to eat a handful of meal worms and some days only cabbage but she made it from deaths door and out to the coop she went.
She still had liquid poop for about 2 weeks, though she ate lots and obviously drank lots and acted like she was feeling great she was not gaining any weight back but her feathers started coming in really beautifully and she was working her way back up the pecking order. I started to suspect that her compromised immune system may have allowed a parasite problem so since I try to go about things naturally I put some food grade D.E. in their food. That seemed to do the trick because soon after she started gaining weight and her poop firmed up really well.
Now we get to the current problem....She is housed in a coop that is 4x8 with 6 other chickens and room for 25 chickens to roost if need be, they are in a run that is about 25 feet by 12 feet that is partially covered and quite posh, they get to semi free range in my hibernating garden that is another 50 x 25 feet most weekends and afternoons. I feed them feed store 18% layer feed unless I happen to find a bag of 20% and then I buy that. They get no more than a couple small handfuls of scratch with meal worms in it each day. I supplement with oyster shells but they much prefer the egg shells that we feed back and there is almost never a day that there are not some available for them to eat still in the run. Part of their run is not covered so they get a decent amount of sunshine, I add a couple tablespoons of Braggs vinegar to their water a couple times a week.They get about 1/2 a cabbage for treat each week and scraps from the kitchen compost bucket so some berries, and bananas and apples usually.
Her P.R. flock mate lays about one egg a week now, everyone else (2 B.O., 2 E.E's and a Brahma) lay 4-5, 5-6, 4-5 respectively. All good sturdy eggs. The hen in question has dropped 2 yolks and one membrane broken around a yolk under her roost in the past week.I have been checking her abdomen each day for size and firmness and all is well so far and she is back to her healthy weight by the way. I am suspecting that she is just not processing the calcium for some reason but any advice is welcome I again apologize for the long post but thought thorough background would be helpful.