Registration form not submitting for all users

[PokerVillain]

Hello,

We are running Vbulletin 4.2.1 and we have an intermittant issue with our registration form, located here: http://linkslistforum.com/register.php

For some users, the registration page and submit is working without issue. However, we are receiving a number of complaints from people who are trying to register- and when they click submit the registration fails (without giving any reason for doing so) and simply refreshes to the same page (http://linkslistforum.com/register.php).

Can anybody shed any light on this?

Thanks,