Hello username not displaying

I'm using the following code on my home page:

<?php
if ($au=$_SESSION['_amember_user']){ // user is logged-in
print "Hello $au[name_f] $au[name_l]!<br>";
print "<form method=post action='/amember/logout.php'>
<input type=submit value='Logout'>
</form>";
} else { // user is not logged-in
print "<form method=post action='/amember/login.php'>
Username: <input type=text name=amember_login size=10><br>
Password: <input type=password name=amember_pass size=10><br>
<input type=submit value='Login'>
</form>";
}
?>

The login works but when the user returns to the home page they don't see Hello User with a logout button.

What am I missing?

Thanks!