do you have the tag in your web.xml file?
Let me try this again :
Do you have the tag
in your web.xml file?
Also, be sure that objects in the session implement serializable.
Yes, I have tried with [code/]distributable in web.xml also. And all objects are serializable too. After analysis I found that sometimes (random behaviour) most of the missing session variables are of type vector, String  and String.
We analyze that this problem happens more after restarting of the server and with the time it becomes less.
This problem also happens in non - clustered enviornment.
If 10 Session Attributes are there in session then only few of them are missing not entirely the session object. This problem is not consistent also if you click back link in Browser and submit it again then it will work fine.
If any one has any idea please help us out.