The action of the form is just the location that the form will post to, it should not have any effect on how the captcha displays on the page. The form must post to whatever page you have your server behaviors on to trigger those server behaviors.
Do you have some links where we can see this? It would be best to be able to see a version of the page with captcha working and a version of the page where it is not working. This will allow us to compare the two.