When you cast a video to Chrome cast is just says:
Casting on [Chromecast Name]
and it does not work. To fix this, close the tab while you are trying to cast it, open a new tab, stop it casting (if it is casting the tab), then close and reopen the tab, cast the tab, then navigate to the video you want to cast. At this point, Google Chrome will probably crash. Close all Chrome windows and start again, it should then work.