Having a problem with onEnterFrame in mx

I am running a test using onEnterFrame in flash mx but its not working for the life of me…
Heres the code:
onEnterFrame=function(){
size_txt.text=test_mc._xscale;
}
I am trying to get the text box to always register the width of the “test_mc” however it will not change from 100 it doesn’t even start at 100! I tried changing the “_xscale” to _xmouse to see if it would change the text and it worked but still not for “_xscale”! grateful for any help!