how to append options in select bootstrap?

My select bootstrap dosn't show the options i append:

This is the index:

<div class="form-group label-floating">
  <div class="row">
    <div class="col-lg-5 col-md-6 col-sm-3">
      <select id="recau_agente" class="selectpicker" data-style="btn btn-primary btn-round" title="Single Select" data-size="7">
      </select>
    </div>
  </div>
</div>

This is the jQuery to insert the options in the select:

var ruta = "https://maxtechglobal.com/vencimientos/arba/conceptos_recaudacion.php";
var $select = $('#recau_agente');
$.getJSON(ruta, function(json) {
  $select.html('');
  $.each(json.data, function(index, value) {
    $select.append('<option id="' + value.concepto + '">' + value.concepto + '</option>');
  });
});