AJAX not working second time

function onQuantityChange(id)
{
    var quantity=$( "#"+id+" option:selected" ).text();

    $.ajax({
        type: "POST",
        dataType: "json",
        url: "cart.php?id="+id+"&quantity="+quantity,
        success: function(data)
        {
            $('.success, .warning, .attention, .information, .error').remove();
            if(data.HTML!=null)
           {     
		//code to update the view on page
           }
               
            
            else
            {
                alert("Unable to get it");
            }
        },
        error: function(){
        alert("Error in event:onselect");
        }
    });
}

for($i=0;$i<count($_SESSION["contents"]);$i++)
{
$sql="select * from products where ID=".$_SESSION["contents"][$i]["ItemCode"];
$con->Query($sql);
while($row=$con->Row())
{
echo "<tr>";
echo "<td class=\"name\">";
echo "<a href=\"\">".$row->Name." ".$row->Unit."</a>";
echo "</td>";
echo "<td class=\"details\"><span class=\"MRP\">Rs $row->MRP</span></td><td class=\"details\"><span class=\"details\">Rs $row->SP</span>";
echo "</td>";
echo "</td>";
echo "<td class=\"quantity\">";
echo "<select id=\"$row->ID\" onchange=\"onQuantityChange('$row->ID')\" >";
for($j=1;$j<21;$j++)
{
if($j==$_SESSION["contents"][$i]["quantity"])
{        
echo "<option value=\"$j\" selected>$j</option>";
}
else
{
echo "<option value=$j>$j</option>";    
}
}
echo "</select>";
echo "</td>";
echo "<td id=\"$row->ID\" class=\"total\">".$_SESSION["contents"][$i]["quantity"]*$row->SP."</td>";
echo "<td class=\"remove\">";
echo "<img src=\"Stylesheets/image/remove.png\" >ID')\" title=\"Remove\"/>";
echo "</td>";
echo "</tr>";
}
}
}
else
{
echo "contents array doesnt exists.";
}
echo "</tbody>";
echo "</table>";
echo "</div>";
echo "<div class=\"cart-totals\">";
    echo "<table class=\"right\">";              
                echo "<tr>";
              echo "<td class=\"total\" width=\"200px\" align=\"right\">";
               echo "<h3>Shipping</h3>";
               echo "</td>";
               echo "<td class=\"total\" width=\"70px\" align=\"right\">";
               echo "Free";
               echo "</td>";
               echo "</tr>";
               echo "<tr>";
               echo "<td class=\"total\" width=\"200px\" align=\"right\">";
               echo "<h3>Total</h3>";
               echo "</td>";
               echo "<td class=\"total\" width=\"70px\" align=\"right\">";
               echo "<h3 id=\"FinalTotal\">Rs ".$_SESSION["total"]."</h3>";
               echo "</td>";
               echo "<td class=\"total\" width=\"40px\">";
               echo "</td>";
               echo "</tr>";
               echo "</table>";
echo "</div>";
echo "</div>";

echo "<div id=\"BackForthButtons\">";
    echo "<div class=\"checkout\">";
        echo "<div class=\"cart-modules left\">";
            echo "</div></div><div class=\"right\"><a class=\"button\" href=\"checkout.php\">";
        echo "<span class=\"proceed\">Checkout</span></a></div>";
echo "<div class=\"right\">";
echo "<a class=\"button\" href=\"http://www.cartup.in\" style=\"background: #454545; margin: 2px 5px;\">";
echo "<span class=\"continue\">Add more items</span>";
echo "</a>";
echo "</div>";
echo "</div>";
echo "</div>";
echo "</div>";
echo "</div>";
echo "</div>";
echo "</div>";
echo "</div>";