BUFFER ModelListener doesn't work

hi, i have a prob:

<script type="text/javascript">
var player = null;
function playerReady(obj) {
player = document.getElementById(obj.id);
addL();
};
function addL() {
if (player) {
if (typeof player.addModelListener == 'function') {
player.addModelListener('STATE', 'stateL');
player.addModelListener('BUFFER', 'bufferL');
}
} else {
setTimeout('addL()', 100);
}
}
function bufferL(params) {
alert('DEBUG BUFFER PERCENTAGE '+params.percentage);
if (params.percentage == 100)
alert('delvid buf');
}
function stateL(params) {
if (params.newstate == 'COMPLETED')
alert('delvid sta');
}

window.onload = function() {
var so = new SWFObject('a.swf', 'pl', '720', '380', '9');
so.addParam('allowscriptaccess', 'always');
so.addParam('allowfullscreen', 'true');
so.addParam('flashvars', 'config=a.xml');
so.write('a');
};
</script>

the 'DEBUG'.. alert never shows, so the bufferL() never called
why and how can i solve?

thanks