This is caused in many cases by the server not replying with an Accept-Ranges header. For an obscure, only-Google-developer-relevant reason (the FF folks don’t seem to be so offended by a server replying without an Accept-Ranges header), they refuse to implement seeking when such a header is not provided. You can add one very easily (at least for testing) by using an nginx proxy and adding proxy_force_ranges on; (server or location both work).