Sateesh,
It's hard to say without seeing your code. Somethings I usally check are these.
1. Am I using the same name for the User object in each place I am pulling it out? Remember even with attribute names, caps rules apply. User and user are two different attributes.
2. Am I adding the user object to the session and getting then putting it back each time. While
Java is reference lang, I find it's always better to implictly put and get attribute objects on the session.
3. Are you using frames, or popups that create new client instances? The server maintains Session connection by means of cookies related to the instance of the client browser. In some cases new popups create new broswer instances on the client computer, so when a
servlet is ran from that new window, there is no session connected.
Hope these help, short of this, we're going to need some code and more concrete examples or concerns.