Anna, I’m not able to answer your question without a lot more information.
Is Medium saying you have exceeded the number of sections you can use? Or are you limited in the total number of stories?
Medium allows a maximum of 25 stories per section. You can increase or decrease the number just to the right of the place where you decide whether you want to list your stories as featured, latest, trending or tagged.
When you say recently published stories are not showing up, you need to check for the obvious stuff first. Did the stories get submitted and accepted to the publication? If they are supposed to go under a certain navigation tab subject, do they have the correct tag corresponding to the tag you used to define your tab? If they are feature stories, do you have empty spots so you can feature them? If they don’t show up in the pull down menu, you’ll have to type in the first letter of the story name to search for your stories. I noticed that little search feature yesterday and I had never seen it before.
I hope this helps.