Codeigniter Multi-level Inheritance
===

I am having problem in using multilevel inheritance as follows.

I have a top level controller that extends CI_Controller class

    class Application extends CI_Controller
    {
    }

A controller named 'Site' and 'Admin' extends the Application controller as

    class Site extends Application
    {
    }

    class Admin extends Application
    {
    }

And finally class 'User' and 'Guest' extends 'Site' controller

    class User extends Site
    {
    }
    
    Class Guest extends Site
    {
    }

The problem is, in User and Guest controller I am not able to load core libraries such as pagination, form_validation etc. using

$this->load->library('pagination);

But it works when I load the library in Site controller or Application contoller ie. controller that extends the core CI_Controller and it's child controller. When I try to load in grand child it doesn't work. 

Can somebody clarify why this is happening? Thanks...