forms auth resets too soon - properties look correct

I can't figure out what's going on with this forms authentication page.  I checked the cookie that's being set and it shows that the expiration date is in 2 months.  But after about 2 minutes of inactivity, I get redirected to the login page.  I'm on one domain, it's all in the following format; Calendar is a protected directory:
https://www.example.com/adminDir/Calendar/

Here's the only active part of the login page, located in the root of the webserver:

public void Submit_onClick(object s, EventArgs e)
    {
    if (FormsAuthentication.Authenticate(txtUserName.Text, txtPassword.Text))
    {
      FormsAuthentication.RedirectFromLoginPage(txtUserName.Text, true);
    }
    else
    {
      lblMessage.Text = "Invalid login credent ...