bootstrap validator after ajax success

Question

In Bootstrap 3, I've a main DIV #content-page that contains a first #button_1. On #button_1 click, Ajax loads a form on the same DIV, #FORM, that contains some required inputs and a submit button.

$('#FORM').validator().on('submit', function(event) { //do somethings.. });

Why validator() doesn't work?

$('#FORM').on('submit', function(event) { //do somethings.. }); ..instead 'this works fine..

How do I validate this form (with plugin validator)?

PS: plugin validator: http://1000hz.github.io/bootstrap-validator/