when clicking on a button shown by while, it only throws data from the first tour of the while, although click on the second button shown

I am starting in php and I occupy shopping cart with sessions, I show the products through the database with a while and in each while there is a button with the id of the product and when I click on the button , calls a function of js where it receives the id and the pieces and then sends them to another function that uses ajax to send all the data to a php file where I create an array where I keep all the data of the product id, name, price etc then that array I keep in a session "orders" with a subindice equal to the id of the product, the problem is that when I give in add to a product that is not the first one shown, it does not send it, but if it works with the first product shown, I do not know what to do the truth I searched but nothing I give, I leave the code attached

Here I show the products and the button performs the function of js

  <?php
   foreach ($resultado as $registros) {
   $datos=$registros['ID_PRODUCTO'];
  ?>
 <li>
 <img class="imagenesproductos"src="<?php echo 
 $registros['DIRECCION_IMAGEN']; ?>" alt="">
 <h1><?php echo $registros['DESCRIPCION'] ;?></h1>
 <p class="precio">PRECIO: <?php echo $registros['PRECIO']; ?> </p>
 <p class="descripcion"><?php echo $registros['descripcion_producto'] ;?> 
 </p>
 <p>Piezas:<p>
  <input type="number" name="piezas" id="piezas" min="0" checked 
  placeholder="Numeros de piezas">
  <input type="hidden" id="id_producto" name="" value="<?php echo $datos;? 
  >">
  <input type="submit" name="" value="">
  <button type="button" onclick="productos()" name="agregar" id="agregarp"      
  value="" >Agregar</button>
  </li>
 <?php
 }
 ?>

in the function I rescue the values of the product and the selected pieces and I send them to another function that is responsible for sending it to a php file

 function productos(){
      id=$('#id_producto').val();
      piezas=$('#piezas').val();
       agregarpedido(id,piezas);
}

function with ajax

function agregarpedido(id,piezas){
  cadena="id=" + id +
  "&piezas=" + piezas
  $.ajax({
    type:"POST",
    url:"../php/pedidos.php",
    data:cadena,
    success:function(respuesta){
          alertify.success("Producto agregado");
    }

  });
}

php file here I create sessions and I keep the records in session

<?php
  require_once('../funciones/bd_conexion.php');
  session_start();

  $pedidos=array();

  if (isset($_SESSION['pedidos']))
    $pedidos=$_SESSION['pedidos'];
  }


 $id_producto=$_POST['id'];
  $sql=("SELECT * FROM producto where ID_PRODUCTO ='$id_producto' ");
  $resultado=$conn->query($sql);
  $registros=$resultado->fetch_assoc();
  $item=array(
  'id'=>$id_producto,
  'producto'=>$registros['DESCRIPCION'],
  'precio'=>$registros['PRECIO'],
  'piezas'=>$_POST['piezas']
  );

  if (!empty($pedidos)) {
    foreach ($pedidos as $recorrido) {
    if ($recorrido['id']==$id_producto) {
    $item['piezas']=$recorrido['piezas'] + $item['piezas'];
      }
    }
  }
  $item['subtotal']=$item['precio'] * $item['piezas'];;
  $_SESSION['pedidos'][$id_producto]=$item;



 ?>