Progressive video: how to grey out nav buttons

I am using Flash 8. Progressive video, video template from adobe with screens, FLV Playback component. I want to grey out (_alpha) my nav buttons until bytes are loaded to that point so that visitors are not clicking on buttons that won’t work yet. I have the following script on a separate layer, frame 1. The alpha won’t change. What am I missing? thank you in advance:

//***************greying out buttons
slide6_btn._alpha = 10;
slide11_btn._alpha = 10;
slide17_btn._alpha = 10;
slide21_btn._alpha = 10;
slide23_btn._alpha = 10;
import mx.video.
;
var listenerObject:Object = new Object();
listenerObject.ready = function(eventObject:Object):Void {

if (display.bytesLoaded > 2140076) {
slide6_btn._alpha = 100;
}
if (display.bytesLoaded > 3923473) {
slide11_btn._alpha = 100;
}
if (display.bytesLoaded>6063549) {
slide17_btn._alpha = 100;
}
if (display.bytesLoaded>7490267) {
slide21_btn._alpha = 100;
}
if (display.bytesLoaded>8203626) {
slide23_btn._alpha = 100;
}
}