Some very old routers don't support IPv6 but that problem does'nt come up very often. Using A/V software and third party firewalls can also block access or prevent the ping from working correctly.
You should be able to see if both machines can access the IPv6 interface by doing an ipconfig /all on both machines and compairing the results. The physical IPv6 address under the torredo tunneling Pseudo-Interface should be the same. And the first part of the IPv6 address should be similar.
The link below shows you how to ping Ipv6 addresses but I'm not sure if it's possible to do so on the LAN. Never had a reason to ping an IPv6 address either because it usually just works although you can ping a special WAN Google IPv6 address that is set up for testing purposes only. http://www.cyberciti.biz/faq/howto-t...ping6-command/