Are they pullets just starting to lay or have they been laying for a while? Sometimes it takes a pullet a couple of weeks to get the kinks worked out of her internal egg making factory.
If it is older hens it may be something wrong with the hen. Some just don't process the calcium they eat correctly, no matter how much they eat. Often that is genetic.
Sometimes a hen might release more than one yolk a day to start a new egg. If she releases them at the same time you can get a double yolked egg. If they are spread out a bit two different eggs may make their way through that internal egg factory. The hen makes a limited amount of shell material so sometimes there is not enough for the second egg. Are you getting several double yolked eggs? This could be a sign you have a hen releasing too many yolks.
It takes an egg about 25 hours to make its way through the hen's internal egg making factory. Most of that time is spent in the shell gland where the shell is being added. If something happens to cause her to lay the egg before the process is complete that egg shell can be thin or non-existent, just a membrane. These eggs are usually white if it is a brown egg laying hen.
How many low-calcium treats are you feeding them? Maybe she is filling up on those treats and not eating enough regular feed to get the calcium she needs. Most hens have the instinct to eat oyster shell if they need the extra calcium for the shells but this instinct can be stronger in some than others. Maybe she is not eating enough calcium.
Some diseases can cause soft eggs or otherwise weird eggs. If the hens are acting normal and not standing around all fluffed up and lethargic and the other eggs are normal it is probably not a disease.
When I have these types of problems I try to determine if it is a flock-wide problem or an individual hen problem so I know how to approach it. I don't want to hurt the flock because of one hen's individual problem. This sounds more like an individual hen problem than a flock problem but I don't know that for sure. My suggestion is to stop feeding them extra treats if they are getting a lot for a week and see if that helps. Or try to determine which hen it is (that can be hard) and if you want her in your flock. If it is pullets just starting to lay, wait a month before you do anything to see if it clears up in its own.
Good luck!