Next button strangeness with MP4 videos

I have an odd occurence with the Next button in the middle of MP4s playing back. I have around 40 modules, same template, same player, same settings. 

1. First slide is set to play an MP4 video bumper with Automatic advance to the Slide 2-all good there.

2. Slide 2 is another MP4 video (around 45 minutes or so) with only the Next button for navigation.

Sometimes when playing Slide 2, clicking the Next button will move to the next slide-Slide 3.

Other times, clicking the Next button will not work until you get to the end of the slide.

Anyone have an idea why there are two different behaviors? I have checked and re-checked the Player settings, slides settings etc. They are ALL the same 

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