Javascript - $.ajax Json Method Does Not Hit The Web Method?

When this function is hit , it does not call my function in code behind? Why could it be doing this? How can I fix this error.

$(document).ready(function() {
$('[id$=btn_Update]').click(function() {
var reten = $('[id$=txt_Reten]').val();
var i=0;
var selectValues = "";
var ProdID = new Array();
$("#lst_ProdId option").each(function() {
selectValues = selectValues + $(this).text() + ",";
ProdID[i] = $(this).text();
i++;
});
for(var j=0; j < ProdID.length;j++)
{
// alert(ProdID[j]);
}
var params = "{'ProdID':'" + ProdID + "','RetenP':'" + reten + "'}";
$.ajax({
type: "POST",
url: "/ProductPricing/Products/RetenPeriod.aspx/UpdateRetenPeriod",
data: params,
contentType: "application/json; charset=utf-8",
datatype: "json",
success: function(result) {
alert("sucess");
},
error:function(e) {
alert(e.statusText);
// if(errorThrown != null)
// alert(textStatus+ ":"+errorThrown);
// else
// alert("fail");
}
});
return false;
});
return false;
});

This is my webmethod in code behind:

[WebMethod]
public static bool UpdateRetenPeriod(string[] ProdID,string RetenP)
{
for (int i = 0; i < ProdID.Length; i++)
{
update(ProdID[i],RetenP);
}
return true;
}