I had problems receiving emails from contact forms on my mortgage website ;);)
I use formidable forms, it did cost a little but it does what I needed it to do. So to answer the question I had to go into the form that was in question and go to the settings and check two things:
X Load and save form builder page with AJAX
X Submit this form with AJAX
Wa lah it worked.
Not sure if it is the same in the free version but it might be.
