We've had reports - mostly from schools using Chromebooks - that the videos won't play. At least some of these reports appear to be due to firewall restrictions.
Workaround: Wait for 90 seconds so the fallback video can start.
If the workaround does not help, please use the Report a problem button in the Help Center and send a Help request. Be sure to include information about the equipment you are using and the network response data. This should help us rule out some issues, and perhaps pinpoint the cause.