Recaptcha Doesn't Work When posting new thread

[rapung]

Dear administrator

I have a problem with my vbulletin version 4.1.3.

I have activated recaptcha for "post a new thread" in this way :

1. Settings > Options > Human Verification Options > Register (checked), Contact Us (checked), Post (checked), Recover Lost Password (Checked), Search (checked)
2. Settings > Human Verification Manager : Recaptcha (Checked), Public and Private Key provided by Recaptcha (Filled).

It works when users do registration, recovering their lost password, contacting us, and searching. But it doesn't work when user post a new thread.

Are there any instructions I missed ?

Thank you