I had 2 youtube videos in my presentaion that wouldn't play. I deleted them in the presentation and have deleted the cache twice. However the presentation keeps showing a blank screen during the time I originally scheduled the videos to play.
If your using android you might have to clear data and put in all your settings again. Youtube on android will save data to the data cache. Something i cant control. Thats a google thing. If its a PC then you might need to reboot after clearing the cache. I have seen youtube currupt its own data streams and sometimes rebooting after a cache clearing will fix the issue.