Login.php problem - please help

I'm really struggling to get the login.php page to work.
Register is fine - I can see the registered user in my mysql database.
However, when I try and register that user I don't get my members.php page as I'd expect.

The start of my code in login.php looks like this:
<?php
ob_start();
require_once($_SERVER['DOCUMENT_ROOT'].'/db_connect.php');

if(isset($_SESSION['username']) && isset($_SESSION['password'])) {
//REDIRECT TO USERS PROFILE...

header("Location:http://www.mysite.co.uk/index.php?exec=members";

ANY IDEAS??