Is the
ViewState
property is Enabled? If not enable it.
If it is enabled you can save the selected index in a Session and reassign it later.
OnSelectedindexchanged event assign something like
Session["ddlAdminValue"] = ddlAdmin.SelectedIndex;
On PageLoad() you can do something like;
if(Page.IsPostBack)
{
if(Session["ddlAdminValue"] != null)
{
ddlAdmin.SelectedIndex = Convert.ToInt32(Session["ddlAdminValue"]);
}
}