wp_schedule_single_event troubleshooting

Hi,

I am trying to schedule the following event
>>>>
add_action(‘zipcreateevent’, ‘createzipfordownload’, 10, 2);

$ret = wp_schedule_single_event(time()+10, ‘zipcreateevent’, array($list, time()));
if (is_null($ret)) {
$user_msg = “success”;
}
else {
$user_msg = “failure”;
}
<<<<
In the function createzipfordownload() I am creating a zip file. But I dont see any file being created. Also the function – wp_schedule_single_event – returns success (null).

Please tell me how to go about debugging the problem.