If it does not work and the theme is older (before 1.7), then there might be the input hidden security code missing.
I ran a couple of times into this issue.
For instance for a custom login field I need to add:
<input type="hidden" name="dologin" value="1">
<input type="hidden" name="code" value="'.qa_get_form_security_code('login').'">
For your specific case:
Find the modification by the custom theme and add the code above with the appropriate qa_get_form_security_code().