Creating a plugin; create table on activation

I have the following code setup to create a table when the plugin is activated, but no table gets created.

Notes:
– When i copy the SQL direct to phpmyadmin it works
– The database user does have permission to create tables.

$scores_table_name = $wpdb->prefix . "scorepredictions";
$scores_db_version = "1.0";

function scores_install () {
   global $wpdb;
   global $scores_db_version;
   global $scores_table_name;

   if($wpdb->get_var("show tables like '$scores_table_name'") != $scores_table_name) {

      $sql = "CREATE TABLE " . $scores_table_name . " (
	  id mediumint(9) NOT NULL AUTO_INCREMENT,
	  post_id int NOT NULL,
	  user_id int NOT NULL,
	  home_team VARCHAR(255) NOT NULL,
	  away_team VARCHAR(255) NOT NULL,
	  home_score int NOT NULL,
	  away_score int NOT NULL,
	  UNIQUE KEY id (id)
	);";
      require_once(ABSPATH . 'wp-admin/includes/upgrade.php');

      dbDelta($sql);

      add_option("scores_db_version", $scores_db_version);

   }
}

register_activation_hook(__FILE__,'scores_install');

Any ideas as to what i’m doing wrong?