Why is FormsAuth....Authenticate() Always False?

  I am using a valid username and password (verified in the debugger), but the if statement is never true and the Response.Redirect never executes. I just don't understand why???

Code:
protected void Page_Load(object sender, EventArgs e)
{
SetFocus(My_Login); // Using the Login control for some users

if (!IsPostBack)
{
string UserName = Request.QueryString["UName"];
string Password = Request.QueryString["PW"];
if (UserName != null)
{
if (FormsAuthentication.Authenticate(UserName, Password))
{
Respon ...