Preloading problem

I made a small presentation in flash where the first scene has the preloader stuff and the second one has the presentation. In the presentation I use the attachMovie to load several movieclips which contains the scrollBar component from Flash. My problem is that when I simulate dowloading with the preloader appears when is already 50%, however if I deleate the movieclips that contain the scrolBar from the library and simulate it again it works fine. Do anyone knows what could be happening?
Thanks in advance