Session fails to maintain after page redirect

Session fails to maintain after page redirect

I have been beating my my head over this. My code is virtually identical
to other projects where this DOES work. Here is how I do it:
session_start();
set_up_session($username);
redirect_to('index.php');
And the two functions:
function redirect_to($location=null) {
if($location!=null) {
header("Location: {$location}");
exit;
}
}
function set_up_session($username) {
session_start();
$_SESSION['user_id']=$id;
$_SESSION['logged_in']=true;
$_SESSION['username']=$username;
}
if I comment out the redirect and echo any of the $_SESSION var's, the var
reads correctly. But after the redirect, the session ends.
This is what's on the next page.
<?php if (!isset($_SESSION['logged_in'])) { ?>
// do stuff <-- this is what gets shown showing session is no longer active
<?php } else { ?>
<p>Hi, <?php echo $_SESSION['username']; ?></p>
<?php } ?>