This happens because of the following reasons - 

• The date for which the sign-up sheet was created is a past date than the current date. 
  • Please note that the if the sign-up sheet was created for a past date than the current date then it will no longer show in the Sign Ups tab.  However, you can find it in the Home feed. Clicking on the sign-up post in the feed will give you the details. 
• You are inside the class page and do not see it in the SignUps tab. But can see the sign-up sheet in the Sign Ups tab of Show For All Groups
  • This means that you created the sign-up sheet in your personal calendar and not the class calendar and hence it is not showing in the class sign ups tab. It is showing in the Show for all groups > Updates > Sign-ups tab. Unfortunately, we cannot move that sign up into the class calendar. Going forward please make sure you pay attention to the calendar field during the creation of the sign-up sheet.