su error

[Kaushal Kumar <kaushal kumar wipro com>]

Hi,

I have Redhat Linux 7.1 installed on my machine, running Gnome desktop.
When i try to use su in the terminal, its gives me an error,

[kaushal kaushal kaushal]$ su
Password: 
su: incorrect password
[kaushal kaushal kaushal]$ 

The same password works fine when I am in the not running a desktop. 
This error was not coming earlier. I am not sure what I changed that it started coming.

If someone has any clue, please respond.
Thanks