There could be several reasons:
The video keeps stopping constantly and the quality is poor
Please refer to this article: The video is of poor quality and stops at regular intervals, why?You can watch part of the video but then get an error message
Take note of the error message and refer to this article: Why do I see an error message when I launch a video?
There are several steps you can follow to try and solve the issue.
Please refer to this article: I’m having trouble streaming a video—what can I do?