Some videos don't play in uploaded course

I have a project in which there is a separate video on five consecutive slides. The videos are short and highly compressed and the total file size for the project is under 90mb. When I preview the course on Adobe Prime the videos on the first two slides play with no problem. The remaining 3 videos will not play. There is a black frame with a line through the play button in the middle of the screen.

When I output the course to my computer and play through my browser (Safari), the videos all play without any problems. Any idea what may be happening?