Edit: it didn’t update the replies, I didn’t know someone said this already.
Do you offer extra calcium like oyster shell? Often soft shelled eggs are due to the hens needing more calcium. Your birds are young, and if you offer calcium already, it could be that the young hens are still working out all the kinks internally. What type of food do you feed, if you give them calcium, does it look like they’re eating it? I have one hen who is already about 10 weeks, all the other hens lay beautiful perfect eggs every day, but she deposits soft sacs of egg goo every few days. It doesn’t bother me so much, obviously I’d prefer every hen pull her weight egg wise, but the other birds like to eat the soft shelled egg and usually eat most of it before I get it. They have oyster shell which I’ve seen them eating, their food has between 2.5 and 3 % calcium, so I’m guessing she’s just a late bloomer.