To aid in debugging, you can use the find option to see whether or not a user or class has access to an application.
In the applications section, bring up the actions menu (click the three dots) for the app you want to check, then click "Find".
You can search for either a class or a user. Classes can be searched by sourcedId, while Users can be found with either a sourcedId or username. Click search to find the Class or User.
If found, you will see a screen similar to this one. For Users, you will the information about the user, and the classes they are enrolled in and their demographic information. Searching for a Class will give information about the class and the users enrolled in it.