session time out before specefied time in asp.net application - asp.net

Question

I have a web application hosted on IIS 7 running on windows server 2008 R2.Users of my application where able to login the application with default session timeout of 20 minutes which is configured in web.config file and session settings on IIS 7.
Users authentication is done by active directory domain services running on another server, that i implemented using this code.
code:
public bool CheckUserInActiveDirectory(string userName)
{
try
{
string filter = "(&(objectCategory=person)(objectClass=user)(SAMAccountName=" + userName + "))";
string[] propertiesToLoad = new string[2] { "name", "PwdLastSet" };
DirectoryEntry root = new DirectoryEntry(activeDirectoryAddress);
root.AuthenticationType = AuthenticationTypes.None;
root.Username = activeDirectoryName + activeDirectoryDefaultUser;
root.Password = activeDirectoryDefaultPass;
DirectorySearcher searcher = new DirectorySearcher(root, filter, propertiesToLoad);
int count = searcher.FindAll().Count;
if (count >= 1)
return true;
else
return false;
}
catch (Exception ex)
{
throw;
}
}
since my application server (with windows server 2008 R2) is joined to Domain Group, Users session is null, less than 20 minutes and the session time is not equal for other users, and is changed every time for each user.
Finally the user is redirected to login page.Is there any body who knows the reason and guide me how to resolve the issue.
code :
protected void Page_Load(object sender, EventArgs e)
{
if (Session["UserSession"] == null)
Response.Redirect("LoginPage.aspx");
}