AutoCompleteExtender Works Only After Postback

The AutoCompleteExtender works only after there is a postback. The first time the page loads, and user types in the textbox, it doesnt work. But after there is a postback like checkedChnaged, buttonClick etc, & user types now, the suggestion list shows up. Its like the extender is activated only after postbacks.

ASPX:

<asp:TextBox ID="txtSearch" runat="server" OnTextChanged="txtSearch_TextChanged" 
 onchange="itemSelected();"></asp:TextBox>
<asp:AutoCompleteExtender ID="ace" runat="server" TargetControlId="txtSearch" 
 MinimumPrefixLength="2" EnableCaching="true" CompletionSetCount="10"  
 CompletionInterval="0" ServiceMethod="GetCodes" UseContextKey="True"
 CompletionListCssClass="autoComplete"
 CompletionListItemCssClass="autoCompleteItem"   
 CompletionListHighlightedItemCssClass="autoCompleteHL"
 >
</asp:AutoCompleteExtender>

CS:

[System.Web.Script.Services.ScriptMethod()]
[System.Web.Services.WebMethod]
public static string[] GetCodes(string prefixText)
 {
   CountryBLL objCountry = new CountryBLL();
   return objDAL.GetCodes(prefixText);
 }