This issue happens occasionally but I have not found how to make it reproducible.
With the latest iOS 7 beta, going into the application and having it refresh it shows the number of unread stories. Pressing ‘All Stories’, occasionally open the the list of stories with fewer stories than there should be (even fewer than fitting the whole screen). Going back to the previous screen and doing a manual refresh, then pressing again the ‘All Stories’ button fixes this and all unread stories appear.