Unable to expand movie clip inside of scroll

I inherited a .fla file with a scrolling thumbnail image movie clip that moves horizontally inside of a "scroll" behind a mask, with the use of right and left buttons. < > The thumbnail movie clip is much wider than the movie's dimensions. I think it was made in Flash 5. I added about 30 new thumbnails to the movie clip for my client, but now it won't expand anymore. Before, all I had to do was put the thumbnail into the edge of the clip and it would expand to include it. I can get the invisible right and left "scroll" movie clips to scroll even beyond the movie clip by adjusting the numbers in the ActionScript for those two instances. I am not a "programmer type" so this probably is hard to understand. If I try to change the width of the movie clip in the properties box it comes up with an "invalid size and position" error. Actually I think all it would do is stretch-distort the clip anyway, which is not what I want. (if I put in a lesser number it squishes the clip). I just want more space right and left.

Any ideas?
-Sue