There can be several reasons that explain why a video is encountering issues:
1 - Caches
Remember to empty your browser's caches.
2 - Internet connection
Check your bandwidth, a poor Internet connection hinders launching/playing videos.
Click here to check your bandwidth: http://www.speedtest.net/fr/
3 - Your browser
We recommend that you use Google Chrome for optimum use of the platform.
4 - Update Adobe Flash Player
Read this article for more details.