PHP Session variables aren't carrying over
===

I'm having some trouble with a user login that suddenly stopped working. I've got a session_start(); at the top of every page that needs it, but the session variables I've declared seem to have just stopped carrying over to the login pages.

I can also disable the redirect and print_r($_SESSION) on the login page, which will print the appropriate variable.

Here's the code for the login page:

    <? session_start(); ?>
    <?php include($_SERVER["DOCUMENT_ROOT"]."/_/includes/head.php"); ?>
    <?php 
    if (isset($_POST['submitted'])){
    			$username = $_POST['username'];
    			$password = $_POST['password'];
    			function enc($string){
    			$salt = "2@xp";
    			$hash = sha1(md5($salt.$string)).md5($string).sha1(md5(md5($string)));
    			return $hash;
    			}
    			$pass = enc($password);
    			if($username&&$password){
    				
    				$query=mysql_query("SELECT * FROM `users` WHERE `username`='$username'");
    				$numrows = mysql_num_rows($query);
    				if ($numrows!=0){
    					while ($row = mysql_fetch_assoc($query)){
    					$dbusername = $row['username'];
    					$dbpassword = $row['password'];
    					
    					//check to see if they match
    					if ($username==$dbusername&&$pass==$dbpassword){
    					echo ("You're in! <a href='index.php'>Home</a>");
    					$_SESSION['username'] = $dbusername;
    					session_register("username");
    					header("Location: index.php");
    					}
    					else echo ("Incorrect Password");				
    					}
    				}
    				else header("Location: login.php");
    	
    			}
    			else echo ('Login Failed');
    }
    
    ?>

And here is the code for the page to be redirected to at login:

        <?php
    	session_start();
    	if ($_SESSION['username']){
    	echo "<section id='login'><a href='logout.php'>Logout</a></section>";
    	$user = $_SESSION['username'];
    	}
    	else header("Location: login.php");
    ?>

I get kicked back to the login page every time. Any help would be awesome. Thanks!