If some videos play for you, and some do not, it is likely a file size download limitation of your network connection.
A way to diagnose this is to look at the running time of the effected videos.
If shorter videos play, while longer videos do not, than file size limits on your network are the likely culprit.
The longer a video is, the larger its file size will be.
To get around this problem you should toggle the "HD" button on the video player, which will load the "standard definition (SD)" video. Our "SD" videos are much smaller in file size by sacrificing some visual video quality.