I have a slideshow with a mix of still images and .mp4 video files.
If I have a sequence ... Still A Still B Video C.mp4 Still D Still E ... then in Manual Slideshow mode image Still E will appear as a blank image on the screen. The same thing happens when the next .mp4 is shown - ie the second still image after an mp4 is replaced by a blank screen. If I switch to Automatic Slideshow mode the slideshow plays correctly.
I have tested this with over ten different slideshows with the same results - and it has happened in Lightroom 4, 5 and now 6 on a Mac Pro running El Capitan (though it occured also in with older operating sytems).