Soft Egg Shell with no clear cause

[Mregg]

I own three chickens, two of which have now started laying either soft shell eggs or I discover a broken egg from where they were perching in the coop over night. This started about 2 months ago out of the blue with no clear cause. Before this it would be an egg a day from each chicken and the chickens themselves are only 2 years old (Amber hens). I've followed all guidance online, no signs of infection, infestation or poor health. They all seem very happy and are very social with no signs of bullying. They're calciumed up to the max with Oyster shells and supplement in their water. Being fed layers pellets and have been since the day I've got them (winter months supplemented with corn mix)
The coop I keep to a good standard and have recently added a egg rollaway system to rule out one of them pecking eggs, causing others stress.

I'm very much at a loss and Google isn't proving much help. If it was the one chicken I'd be inclined to visit the vet.. but two at the same time? Maybe I've missed something. Any help would be greatly appreciated

 

[Mallards and Quail]

Do they have constant access to your calcium supplements like the oyster shells? If so how do you give it to them separately from their food or with their food?

 

[BigBlueHen53]

How old are the chickens?

 

[BigBlueHen53]

How old are the chickens?

Never mind, I found it.

 

[Mregg]

Do they have constant access to your calcium supplements like the oyster shells? If so how do you give it to them separately from their food or with their food?

I have oyster shells mixed in with their food and zolcal-f mixed with their water

 

[Mallards and Quail]

While it’s not common it’s possible to have to much calcium. If the calcium is in very plentiful amounts it can cause buildups leading to soft shell eggs. The eggs won’t be getting the right amount of nutrients they need and your hens may face some health issues caused by the calcium buildup. Have you noticed any strange behaviors?