Masking Problem - Shape Tween Keyframe

I am updating the portfolio section of this website:

In the process of adding some new items to this portfolio, a masking effect I am using has stopped working. Using the debugger and making changes to the layers and my action scripts I seem to have isolated the problem to a certain key frame.

The mask is animated with a motion tween. One of the layers it masks is a movie clip that loads an external .swf file with photos in it. For some reason when the tween reaches the final keyframe of the transition, the movie clip with the loaded pictures dissappears. If I stop the animation one frame earlier there is no problem, but the mask animation is, of course, not complete. In the earlier version I made (linked to above) I never ran into this problem even though as far as I can see I am doing the very same thing.

So has anyone heard of any problems with masks, shape tweens, keyframes, and dynamically loaded movie clips? Thanks for any help you can offer.