Video Demo slide missing video in HTML5 preview

[cassbg]

I have a 60-slide Adobe Captivate 2019 project that includes two video demo slides. Audio was added to the slides separately. The first video demo slide plays perfectly in Captivate when previewed using the timeline and when previewing in HTML5 in Chrome. The second video demo slide works fine if played on the timeline; however, when previewing the slide using HTML5, the video is missing from the slide while the audio and everything else plays as it should. What could be going on here? I have cleared the cache for both the project and my browser. Any solutions would be much appreciated. 

[Lilybiri]

There is no Preview method possible in the Timeline.  There is a Play slide, but that is not a Preview at all, it is just meant for synchronization.

Moreover if the video demo slides are in a non-responsive project, the only trustworthy Preview method is F11, Preview HTML in Browser. Since that is not working, something is awry with the second video demo slide. Can you re-encode the video with the Adobe Media Encoder to be sure that it uses the correct codec?