Feed solution to a chronic soft egg layer.

Do you provide a separate calcium source, free choice? Even on layer feed, some birds will need more calcium, every bird is different and needs vary. You should provide oyster shell in a separate feeder all the time, those that need more will take what they need. I mix my saved crushed egg shells in with the oyster shell in a 50/50 mix, they like it that way.
How old is she and has she always laid like this? She also could have a malfunctioning shell gland, nothing you could do about that.
Here is more info:
https://blog.cacklehatchery.com/why-are-my-chickens-laying-soft-eggs/

 

Stress and overcrowding can cause shell issues also. If you are giving lots of treats (means anything other than regular feed) then I would stop that for a time and see if you get improvement. It may just be an issue with this single bird, some birds have more trouble with certain foods than others. Make sure your feed is good quality and fresh (check the mill dates on the bag, buy the freshest you can get). There are also some virus's that can impact egg formation, you would likely be seeing it in more than one bird though. Here is more info:
https://countrysidenetwork.com/daily/poultry/feed-health/calcium-supplements-for-chickens/

 

If you are feeding a seed type diet, it's very possible that one or more birds eats the yummy stuff and leaves the rest, causing malnutrition issues. A quality fresh crumble with oyster shell on the side and few treats might help, if that's what's happening.
If there are no problems with diet, and adding another calcium source doesn't fix it, then it's her, and maybe unfixable.
And your males shouldn't be eating a layer diet. How about an all-flock feed instead?
Mary

 

Eggshells are only available for a short time and they quickly pass through the digestive system. Oyster shells stay in the body longer and the calcium is available for much longer. Oyster shells should always be available.

Most soft shelled eggs are from stress. Is she being harassed? Stressed out by something? Things like noises, too much heat, not enough water or food?

If it's a chronic issue that doesn't resolve itself there may be something wrong with her shell gland.

 

There is almost a certainty that any hen who lays a soft shelled egg 90% of the time is not going to respond to a different feed regiment. For the health of your flock any hen like this on should meet the axe or become meat. Not doing so only compromises the health of the rest of your flock.

Do you let your children lick the floors in public restrooms? Well keeping a chicken with the aftermath of an infectious disease is no different.