Kenapa penghitungan otomastis tidak berlaku pada semua menu

<style>
	input.quantity {
		padding: 10px;
		width: 120px;
	}
</style>
<div class="container-fluid">
          <div class="card-header">
            <h1 align="center">Detail Keranjang</h1>
            <input type="hidden" id="id_menu" value="id_menu">
          </div>
          <table class="table table-hover table-striped table-bord text-center">
		<thead>
			<tr>
				<th>NO</th>
				<th>PRODUCT</th>
				<th>QTY</th>
				<th>HARGA</th>
				<th>SUB TOTAL</th>
			</tr>
		</thead>
		<tbody>
				<?php

				$no=1;
				$total = 0;
				foreach($this->cart->contents() as $items) :
					$subtotal = $items['qty'] * $items['price'];
					$total += $subtotal;
				?>
					<tr>
						<td><?php echo $no++ ?> </td>
						<td><?= $items['name'] ?></td>
						<td><input class="quantity" id="qty" type="number" value="<?= $items['qty'] ?>"></td>
						<td><input class="quantity" id="harga" type="number" disabled value="<?= $items['price'] ?>"></td>
						<td><input class="quantity" id="subtotal" type="number" value="<?= $subtotal ?>"></td>
					</tr>
				<?php endforeach; ?>
				<td colspan="3" align="right" class="strong-text">TOTAL</td>
				<td><input class="quantity" id="total" type="number" value="<?= $total ?>"></td>
			</tbody>
	</div>
  </table>
  <div align="right">
		<a href="<?php echo base_url('customer/keranjang/hapus_keranjang') ?>"><div class="btn btn-sm btn-danger">Hapus Keranjang</div>
		<a href="<?php echo base_url('customer/dashboard') ?>"><div class="btn btn-sm btn-primary">Lanjutkan Belanja</div>
		<a href="<?php echo base_url('customer/keranjang/pembayaran') ?>"><div class="btn btn-sm btn-success">Pembayaran</div>
		</div>
</div>
</div>

<script>

$("#id_menu").change(function() {

    class = $('#id_menu').val();

    $.ajax({

      url: 'http://localhost/ap_quiznos/customer/keranjang/getubah',

      data: {

        id: id,

        qty: $("#qty").val()

      },

      method: 'post',

      // dataType: 'json',

      success: function(data) {

      }

    })

    var qty = $("#qty").val();

    var harga = $('#price').val();

    var subtotal = $('#subtotal').val();

    $('#subtotal').val(qty * harga)

    $('#total').val(qty * harga)

});

</script>