Session Time Out of Web application

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C#3.5

hello,

I am developing web application in asp.net with c# in which
I am setting session on button_click.
after some time(hardly 20-30 second) I click any one of link.
my session is lost and I am not getting data related to that session on next page
session timeout is 20 in my web.config I have also increase timeout but still it creating problem.
[ it works fine on localhost but after uploading on server it creates problem ]

Please reply me it is very urgent.

thanks in advance.