This example shows a parent User Story with child User Stories but this article applies to any Parent/Child relationship.
A new feature was recently added to the Quick Details Pages (QDP) which shows children in closed projects. Please check QDP as a first step. If that does not show the missing stories, please proceed with the steps below.
The Test Cases tab may also show a different value on the tab if there are Test Cases associated with a child user story. Please see TEC1612457 for more info.