Paypal ipn response

Below is my form which I sent to pay from action script.

        //var byteArray:ByteArray = price.
        var url:URLRequest = new URLRequest("https://www.sandbox.paypal.com/cgi-bin/webscr");
        //url.data = new URLVariables开发者_运维技巧();
        var obj:URLVariables = new URLVariables();

        obj.business = 'thatfl_1316474816_biz@hotmail.com';
         obj.no_note = '1';
         obj.action = 'paypal';
         obj.cmd = '_xclick';
         obj.notify_url = 'http://www.facebook-books.com/payPal/ipn.php';
        obj.quantity = '1';
        obj.item_name = 'some name here';
        obj.rm = '2';
        obj.currency_code = 'AUD';


        obj.desc = 'Customized Calendar';
        var price:Number = 19.44;
        var tot:Number = price;

        obj.amount =  price.toString();
        obj.total = tot.toString();



        url.data = obj;


        //url.method = URLRequestMethod.POST;

        navigateToURL(url, "payPalWindow");

The problem is every thing works fine except the last part where we expect paypal to send us the IPN notification at the specified address on our profile which is ipn.php and it contains the normal copy pasted code from the paypal.

This file never gets called.

Can any one throw some light what we are missing?