Why Override Are Not Working In 1.6.1.4

[kerintis]

I have created override for /override/classes/order/order.php it works on 1.6.1.3 but for some reason its not working for 14 version. 

<?php
Class Order extends Ordercore
{
public static function generateReference()
{
    $last_id = Db::getInstance()->getValue('
        SELECT MAX(id_order)
        FROM '._DB_PREFIX_.'orders');
    return str_pad((int)$last_id + 1, 9, '000000000', STR_PAD_LEFT);
 
}
}

Whats wrong here ? 

[NemoPS]

Because on 1.6.1.4 they are disabled by default in the performance preferences. Check that out