IntelliSense XML Comments only Works in Original Source File?

I can't seem to get XML Comments to work completely with IntelliSense.

For example,

///<summary>
///My description of member function
///</summary>
int Class::FooFn() {}

These work great within whatever original file I've entered them, and IntelliSense happily shows my summaries, parameter descriptions, etc..  But when I try to use those member functions in other files (that's the whole point), IntelliSense doesn't show any of my XML comments, and just shows the default information.  

I'm using Visual Studio 2012 Express for a Desktop C++ project.  The project does create a centralized xml file automatically, and it does build and run fine.