Symfony2 Form becomes invalid on submit when I am trying to append new elements in form

I have added a button Add section in the form. when a user click that button it opens a prompt window. Prompt window ask for section id After that it makes ajax call to get the section data from database and creates section form with the respective fields Then I am appending that subform(i.e section form) to the existing form. But when I am submitting the form symfony is throwing error that form is invalid. So basically symfony form does not accept the newly created subform appended to existing form

This is how I am adding button to the form in symfony controller

 $formBuilder-> add('new_sections','hidden',array())
  ->add('add','button', array(
        'label' => 'Add Section',

    ))

in twig file, Jquery is appending the new fields to the existing form on button Click

 $('#form_add').click(
        function(){
            var section_id = prompt("Please enter your Section ID");
            console.log(section_id);
            $.ajax({url: url, success: function(result){

                var new_section_form ='' +
                        '
'+
                        ''+result['section']['section_id']+''+
                '
'+
                 '       
Weight 
'+
                '
label 
'+
                '
Active 
'+
                '
Link 
'+
                '
events_max 
'+
                '
events_min 
'+
                '
module
'+
                '
module_icon 
'+
                '
module_id 
'+
                '
apicall 
'+
                '
template_static 
'+

                '<input type="hidden" id="form_'+result['section']['section_id']+'_changed'+result['section']['section_id']+'" name="form['+result['section']['section_id']+'][changed'+result['section']['section_id']+']" class="changedField" value="2">'+
                 '       
Delete Section No. '+result['section']['section_id']+'
'+
                '</div>'+
                '</div>'


                $('.SectionForms').last().parent().after(new_section_form);
                $('#form_new_sections').val($('#form_new_sections').val()+" "+result['section']['section_id']);

            }});
        });

But when I am submitting the form its not entering in form valid block

 if ('POST' === $request->getMethod()) {

        $form->handleRequest($request);

        if ($form->isValid()) {
        // not entering in this block
        }
        }