Web Forms :: SelectedIndexChanged Event Of Drop Down List Not Working?

i n using java script function to showalert msg .function is as follows

function ShowAlertForScaleResolution()
{
var ddlScale= document.getElementById('<#=ddlScale.ClientId #>');
var ddlResolution= document.getElementById('<#=ddlResolution.ClientId #>');
if(ddlScale.value=="0" || ddlResolution.value=="0")
{
alert("Please select valid Scale and Resolution");
}
return false;
}

my dropdown source code is--

<asp:DropDownList ID="ddlBlockName" onchange="return ShowAlertForScaleResolution();"
runat="server" Width="150px" AutoPostBack="True" OnSelectedIndexChanged="ddlBlockName_SelectedIndexChanged">
</asp:DropDownList>

when i m selecting the BlockName withought selecting any item from ddlScale and ddlresolution then i m showing alert msg.

My Problem is that when i m selecting the details of scale and resolution dropdown then its not showing any errormsg. But its not firing the selected index chaged event of Drop down .