Including PHP classes not found further down page

Question

I have had a look around but still can't find out why my Class isn't been found in my page.

I am including the new Facebook v4 API in my Wordpress site - above the header:

//top of site   
include('pageInc/FacebookHeader.php'); 
get_header();  
include('pageInc/innerHeader.php');

But then inside innerHeader.php I am calling the classes defined in FacebookHeader.php:

$fbsession = new FacebookSession( $fbsession->getToken() );
$request = new FacebookRequest( $fbsession, 'GET', '/me' );

And its showing an error:

Fatal error: Class 'FacebookSession' not found in ...innerHeader.php on line 21

If I call the class in the FacebookHeader page it works fine, but just doesn't work inside the actual site. Surely you can just call the class anywhere in the page once it's been included?

The very top of the site in FacebookHeader i am calling the class like:

require_once( 'Facebook/FacebookSession.php' );

Any help would be appreciated