สอบถามปัญหาเกี่ยวกับ Java Script บน php ว่าทำไมถึงทำงานแค่บรรทัดแรกบรรทัดเดียว

<table border = "0" bordercolor = "black"bgcolor = "white" width = '95%' align = 'center'>
<?php
  for($i=1;$i<=15;$i++)
  {

echo   "<tr>";
echo   "<td align = 'center' width = '20%'>$i</td>";
 ?>
<script language="JavaScript">

function fncSum()
	{
		 if(isNaN(document.getElementById('quantity').value))
		 {
			alert('(จำนวนสินค้ากรอกได้แค่ตัวเลขเท่านั้น)');
			document.getElementById('quantity').focus();
			return;
		 }
		 if(isNaN(document.getElementById('discount').value))
		 {
			alert('(ส่วนลดกรอกได้แค่ตัวเลขเท่านั้น)');
			document.getElementById('discount').focus();
			return;
		 }
var prod_price = document.getElementById('prod_price').value;
var quantity = document.getElementById('quantity').value;
var discount = document.getElementById('discount').value;
document.getElementById('one_total').value = (prod_price-((prod_price*discount)/100))*quantity;
	}
</script> 
    <td width = "10%"><input type="text" name="prod_id<?php echo "$i";?>" size="8" 
OnChange="JavaScript:doCallAjax
	(
	'prod_id<?php echo "$i";?>',
	'prod_name<?php echo "$i";?>',
	'prod_price<?php echo "$i";?>'
	);"></td>
    <td width = "44%"><input type="text" name="prod_name<?php echo "$i";?>" size="75" ></td>
    <td width = "10%"><input type="text" id = "prod_price" name="prod_price<?php echo "$i";?>" size="8" ></td>
	<td width = "10%"><input type="text" id = "quantity" name="quantity<?php echo "$i";?>" size="8" OnChange="fncSum();"></td>
	<td width = "10%" align = "right"><input type="text" id = "discount" name="discount<?php echo "$i";?>" size="6" OnChange="fncSum();">%</td>
	<td width = "10%" align = "right"><input type="text" id = "one_total" name="one_total<?php echo "$i";?>" size="6" value = ""></td>
  </tr>
  <?php
  }
  ?>
  </table>