Slide copying between projects

Hi, my colleague and I are trying to copy a scene from one (old) project to a new project, to hopefully fix some errors we can't resolve, so attempting a fresh start. But, when we copy either an individual slide, or a whole scene, (it is a big scene with 60+ slides showing a software process), many of the slides don't copy across properly, and just show some of the images on that slide, and a grey background with the word 'video' in small font in the centre. We'd be very grateful for any help on what's happening if anyone has any thoughts?

Thanks! 

Paul