I can hear the audio but I can't see the slide when I preview!

[hhiple]

I'm going nuts here -  my project played as I wanted it to.  I made a few corrections to some drag and drop slides and now several of the slides are not doing what I want them to do. IN TWO DIFFERENT PROJECTS!!!  I've obviously done something but I can't figure it out.  There are slides in both projects that are fully functional.  This only happened with 2-3 slides per project.  When I preview the project, I can hear the audio but I can't see the material on the slide.  Does anyone have any suggestions?  

[Lilybiri]

Which Preview method do you use? For HTML5 output use F11, Preview HTML in Browser.

I would like to see a screenshot of the Timeline please. One of the requirements of D&D is that ALL drag sources and drop targets need to be available from the first frame of the slide.