All of my forms, both in preview and in their places on the site, will submit (they don't send notifications, which I'm not sure is a related problem or not, but at this point, nothing would surprise me), but the user doesn't get any kind of confirmation that it's been submitted because the page just keeps loading. The user does receive an error message if a field is filled out incorrectly, though, so it seems only to be a submission error.
I've tested this with Ajax activated and deactivated and the results are consistent.
http://www.resamichellemedia.com/contact
http://www.resamichellemedia.com/project-proposal-request