I'm not saying this is definitely what happened, but sometimes, especially when they are first starting to lay, a hen's internal egg factory messes up. Instead of starting one yolk through her system a day, she starts a second. The shell gland does not have enough time to get enough shell material ready, so the second egg can be really soft shelled. It doesn't matter if she is eating enough caclium or not, the shell gland just doesn't have enough time to get enough material together.
Some hens cannot process the calcium properly. It's a genetic thing. Again, it does not matter if she is eating calcium or not. Her system just cannot process it.
Sometimes, a pullet just learning to lay will mess up. It might take a week or even two for her to work out the kinks in her internal egg laying factory. If they are just starting to lay, have patience.
I hate to say it, but some diseases will cause soft shelled eggs.
I don't know what is going on with your hens. There are several different possibilities. With you finding them on the droppings board, I'd guess they are just learning how to lay and need to work out the kinks. But I really don't know.
Good luck!
