Post your javascript code.
A blind guess is : you are trying to setup your ajaxForm before it is actually present in the page.
If you want to run ajaxForm on '#brands_form_update', you have to request for it, insert it in the document, then call ajaxForm. This should give something like :
// $.get(...) sends the request
// $(..).append inserts the form inside the document
// $('#brands_form_update').ajaxForm() sets up the form
$.get('myDynamicForm.php', { ...data...}, function(html){
$('*mySelector*').append(html);
$('#brands_form_update').ajaxForm();
});