PHP values not retrieving from other page

I have this page I am working on which is a sort of a game. From page to page I am passing the user name like this

index:

<form method="GET" action="start.php">
    <label> Name: </label>
    <input type="text" name="username"><br>
    <input type="submit">
</form>

Where I retrieve the name on the second page like this:

<?php
  session_start();
  $_SESSION['username'] = $_GET['username'];
?>

-

  <?php
  session_start();
  if(isset($_SESSION['username'])){
     echo "Welcome: " . $_SESSION['username'];
  } 
  else{                 
     echo "Name is unknown";
  }
 ?>

And also on the third:

    <p>
      <?php 
        session_start();
        echo $_SESSION['username']
      ?>
    </p>

And this code is working just fine. Now I was making an if statement which, when ever you don't enter a name, you won't continue to the next page. I added this code to the first page and this is working for the first page.

<?php
  session_start();
  $_SESSION['username'] = $_GET['username'];
  if($_SESSION['username'] != "")
  {
    header("Location: start.php");
  }
?>

So after adding this, you won't go further unless you do enter a name. But by doing this for a reason I don't know yet and couldn't find, the $_SESSION['username'] = $_GET['username']; isn't working and the names are not passed through

This is the link if you like to play: http://i333180.iris.fhict.nl/site (not finished yet)