Movie Make – edited and packaged mp4 doesn’t play like the original video

I am taking 60 minute recorded webinars and breaking them into small chunks by opening the webinar in Movie Maker, creating the start and end point and then packaging the edited segment as mp4.   Most of the time this works.  However, about every 4th segment, when viewing the packaged mp4 the audio plays but the slides never advance.  So for a 6 minute segment the audio works perfectly, but the slides from the webinar do not advance.   Why does this happen and what do I need to do to fix it?  I've tried repackaging the same segment but end up with the same results. 

Thanks.