Pay Pal IPN通知在沙箱帐户中不起作用

我正在使用贝宝沙箱账户，我有立即购买按钮。我的表单在这里：index.php

<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
     <input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="xxxxxx@gmail.com">
<input type="hidden" name="return" value="http://xxxx.net/success.php">
<input type="hidden" name="cancel_return" value="http://xxxxx.net/cancel.php">
<input type="hidden" name="notify_url" value="http://xxx.net/ipn.php">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="item_name" value="Bike">
<input type="hidden" name="amount" value="12.99">
<input type="hidden" name="a3" value="5.00">
<input type="hidden" name="p3" value="1">
<input type="hidden" name="t3" value="M">
<input type="hidden" name="rm" value="2">
<input type="hidden" name="invoice" value="ADDEdEd3dd3">
<input type="image" src="https://www.sandbox.paypal.com/en_US/i/btn/btn_buynow_SM.gif" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
</form>

这是我的成功。php

<?php
$con = mysql_connect("host","user","pass");
mysql_select_db("db");
if($con == null) {
        echo "Not any connection..";
} else {
        $re = mysql_query("insert into test(value) values('inserted')");
        if($re != 0) {
             echo "Data inserted..";
        } else {
             echo "Error while adding data..";
        }
}
?>

我启用了IPN通知设置和url，还启用了自动重定向url，一切都很好。但是IPN通知不起作用。

知道吗，我只是想ping一下这个页面。