If you are experiencing difficulty viewing a Challenge video first check to see if you are using one of our recommended browsers (Chrome 21+, Safari 6+, Firefox 25+ and Internet Explorer 9+). If you are using a supported browser, check your internet connection speed. Use Speedtest to check your speed. An uploading speed of 2 Mbps is normal.
Additionally, you can try doing a hard refresh of your browser as this helps video playback. Learn how here.