3D Matte Key Image Disappears

I'm working on Avid Express Pro and just encountered a puzzling problem. I was working on a big project with lots of timelines and motion effects created with the Effects Editor. I took a break for a couple of weeks, and now if I modify anything on the timeline, the graphics (with alpha) on the layers above it become invisible, even after I render it out. I can still open the clip and see all the keyframes and parameter values, but the graphic itself remains invisible.

I tried re-importing one of the graphics (a Photoshop file with alpha channel) and was able to successfully put the clip into a new timeline. When I open the clip using the Effect Editor button on my timeline bar, I can change the parameters and everything is fine. One I hit the 3D button in the Effect Editor - thereby changing the effect from "Matte Key" to "3D Matte Key" - the graphic becomes invisible.

I've got about a dozen other projects in the system and this is the only one with this problem. I searched the forums using some of the keywords above and didn't find anything. Does anyone know why this happens and how I can fix it?

Thanks in advance.