Session is closing automatically when i am applying asp,net security and membership

I applied asp.net security and membership to my website. After 1 minute , session is closing automatically. i written session timeout
20 min in web.config also. Why it is closing? .  This is the code i written in web.config file.  please help.
 
<sessionState  timeout="30" />
 <authentication mode="Forms">
   <forms name="form1" loginUrl="login.aspx"  timeout="30" protection="All" slidingExpiration="true"  />
  </authentication>
  <authorization>
       <deny users="?" />
      <!--<allow users="*"/>  -->
  </authorization>
  <!--<roleManager enabled="true"/>-->
  <roleManager enabled=" ...