Anime Studio Masking Problem

[CartoonZack]

Hey. I've been using Anime Studio for a while, but this is really annoying me. I'm working on a "Thumb" movie, where people's faces are put on thumbs. I was able to do this by masking video clips of a person's face and placing it on a video clip of a thumb. The problem I have, is that after a while on the timeline, the masked video clip suddenly shows all of itself, rather than the masked section. Then when I go back to frame 0, the video clip is still unmasked, even though it is masked. A download for it is at http://www.cartoonzack.com/graphics/Raisons.anme

Please someone help me. The program I'm using is Anime Studio 6 Pro.

Oh, and I removed the video clips so it wouldn't lag because of the resolution, but the problem is still there within the program.