Tutorial: Simple Custom Scrollbar

I recently created a simple custom scrollbar using the Flash Tutorial found on this site. My scrollbar (and the one in the tutorial) works fine except when there is enough information (for example: text) inside the movie clip contentMain to make its height greater than 1640. Any information that extends below H= 1640 appears invisible when one tests the movie.

Since I am not very versed in ActionScript, I was wondering if any of you who are could check to see if this is an application bug or a problem with the script in the tutorial.

Thanks.