Masking behaving strangely

I have a movie clip that uses masking. I designed this movie clip by itself for testing and such, and got it working well.

After that, i implemented it into my project, using the same code, etc… creating a new Moive Clip within my main project to put this new MC.

The problem I am having, is with the masking. At first it loads fine, but i use a tween to scroll the object being maked. When I do, that, it does not display correctly. The color changes, and pictures and sprites that I have loaded disappear. If I remove the mask, the whole thing works perfectly, except that you can see all of the things that should be masked (obviously).

How come when I am adding the mask, it screws it up?

Thanks.