OnSelectedIndexChanged event is not firing of the asp.net dropdown list

I have a dropdown that have four predefined transaction types:

<div class="divtd w160 ">
<asp:DropDownList ID="cmbbooktype" runat="server"
class="selectcontrol w140"
OnSelectedIndexChanged="cmbbooktype_OnSelectedIndexChanged">

<asp:ListItem Text="TOPAY" Value="TOPAY"></asp:ListItem>
<asp:ListItem Text="PAID" Value="TOPAY"></asp:ListItem>
<asp:ListItem Text="TBB" Value="TOPAY"></asp:ListItem>
<asp:ListItem Text="FOC" Value="TOPAY"></asp:ListItem>

</asp:DropDownList>
</div>

I need to bind the rest of the controls in the form on the basis of above dropdown so i need to call OnSelectedIndexChanged event but this is
not firing.