Video Closed Captioning Preview Freezing Issue

Hello,

I am working on a Adobe Captivate 2017 project where I am adding closed captioning to a large mp4. Before uploading the closed captioning, I was able to "Preview from Slide" the mp4 with no issues. After the closed captioning was added, the "Generating Preview" freezes at the same point every time. The only preview option that seems to work is the "HTML5". My operating system is MacOS. Can you please tell me why Captivate is freezing? 

Thank you.