Session Expire Problem in Asp.net website

Hi In My System , I have developed a asp.net website

In Which I Have Given the Session.Timeout=20(in aspx page), in IIS Level Also The Session timeout is 20

but in my website the session is expiring in less than 5 minutes ,I don't know why, Please Tell Me How to set The session timeout so that my website session should expire after 20 minutes only

(My sys is connected to four more systems through LAN connection)

(i used the session mode="InProc"  and timeout="20" in web.config)