White dots appearing over image - HTML5

Hey team, I've attached a video to show whats happening. The white dots only appear sometimes, I've making some very simple HTML5 ads and in the exact same process each time, some appear in the timeline and publish just fine and then sometimes they are plauged by these white dots all over the image. Is there something I'm missing here? 

Thank you in advance! 

All The Best 🙂