How much scratch do you feed and do you mix it into the feed or throw it out for them to forage? Do they also have access to granite grit. Crushed oyster shells are just to provide calcium and dissolve in the acidic conditions of the gut. Birds need small hard stones/grit to grind up their food, particularly if you feed them scratch or any vegetation etc. Too much scratch can cause Fatty Liver syndrome which can lead to the liver breaking down and leaching sera into the abdominal cavity, so it may be that and not a reproductive problem.
Edited to add....
Just read your last thread after posting the above and see that she has sadly passed. I'm so sorry

The questions about feed are still relevant however because some birds are more disposed to Fatty Liver than others and if you are feeding too much scratch another bird could be affected. Also if you mix other things into their regular layer feed, it can cause them to bill food out in order to get their favourite bits and cause waste and possibly attract rodents to clean up the spillage and/or cause a dietary imbalance, so always worth reassessing your feeding regime when you have a death.
If you are up for it, you could always open her up and see what the problem was. That way you may gain knowledge that could help your remaining flock. I appreciate it is not something everyone can get their head around but it is really interesting once you overcome the emotions. If you decide to do that, take photos so that we can help you interpret what you find.
Again very sorry for your loss and hope this post doesn't come across as insensitive or critical in any way.