How have you tested this?
If only in the Flash IDE, then it's because the txt file is loaded too quickly and it doesn't have time to register the progress.
The code seems fine, so I think that's the problem.
If you want to test how it would look on a slow connection you can use a webproxy (for instance Charles: http://www.charlesproxy.com/
) to throttle and simulate a different download speed.