Barry,
I think I’ve found the culprit, maybe. I had one slide in the sequence that had a third object – a plane - crossing the screen behind the main image but in front of the background screen, in-between. Removing it solved the problem. How I did it was to pan the entire slide with the plane across the screen over a second black screen so the background of the plane could not be seen. It appears that is not permissible? I would like to use it but I thought you could not key an object or use an object with a transparent background? If not, how would one go about flying a cutout through the screen?
Thanks again for the help