Most of the time, when a video won't play it is due to a slow internet connection. Lowering the quality of the video will fix this and and allows for faster loading speed.

In the bottom right corner of the video player you'll see a settings gear, click on it and you will see a 'Quality' selection.

When you click on that it will bring up a list of different video quality options to choose from.

The lower number will allow for faster loading speed on slower internet connections. (i.e. lowest number for slowest internet speed)