Preloader starts at 50%

Hi,
I followed the tutorial about a preloader http://www.kirupa.com/developer/mx/percentagepreloader.htm
the preloader bar appears when about 50% of the file has been loaded, and then increases up to 100%. How can I fix this?
when I simulate the loading process, the first frame is 8KB and exceeds the loading treshold (200B). Can anybody pls help me?
thanks a lot
Alessandra