Tomcat manager applications are 'by Host' so, if you've set up a host other then localhost in your server.xml file, you may need to install another copy of the manager application.
If this is really important to you, you might want to add a component to your application that shows a little more than the Tomcat manager, which only shows how many sessions per app are open.
I have a sample application on my site that shows how to see more information about each session, like, how long it's been idle, who is logged in, and the last page that that user hit.
http://simple.souther.us Look for SessionMonitor