Noob with a rendering problem after tracking video

Situation: I’ve successfully tracked a two minute .mov file. Camera solve came in at .3xxxx error. Created tracking setup with no issues. Reconstructed “floor” in vid clip window with no problems. I’ve added a plane with a holdout material to act as a table top, and I’m dropping a cube straight down into the sceen. Plane is set as rigid body (passive), cube is rigid body (active). So here’s where it gets odd. When rendering the animation scene for a sanity check, The cube shows up in the animation as transparent. I’m using the standard Blender generated compositing node setup. What am I doing wrong? I know it can be done, because there’s vids all over Youtube. What silly little setting have I missed?

Happy to provide pics, or more data as required.

Thanks, TheSkipinator

Happy to provide pics, or more data as required.
If you’d that automaticaly I wouldn’t have to take this additional time to have to ask for them. Full screenshots and an example blend file showing the problem.

Also please use the support forums for support questions. Thread moved from Blender Discussion