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,

[kh99]

This may be obvious, but just to make sure: if there's something wrong with the form like a required field is not filled in, the form will be reloaded with error messages at the top. Could it be they just aren't noticing the error messages?

[Ba'al]

Are you using a 3rd party Human Verification System like "Are you Human" (http://areyouahuman.com/)after i installed it i had the exact same issues until i disabled a different anti-spammer Mod.