事件执行失败

在我的下拉列表中，SelectedIndexChanged事件没有触发。我设置AutoPostBack="True"，但它仍然没有发射。设置EnableViewState为True或False也没有区别。

下面是我的代码:

<asp:DropDownList ID="ddlSheerName" runat="server" Width="250" AutoPostBack="True"
 OnSelectedIndexChanged="ddlSheerName_SelectedIndexChanged"></asp:DropDownList>
protected void Page_Load(object sender, EventArgs e)
{
    loggedInUserId = Convert.ToString(Session["LoggedInUserId"]);
    if (loggedInUserId == "")
    {
        Response.Redirect("Login.aspx");
    }
    if (Page.IsPostBack == false)
    {
        BindCompanyDropDown();
    }
}
protected void ddlSheerName_SelectedIndexChanged(object sender, EventArgs e)
{
    Bindcolumnname();
}
public void BindCompanyDropDown()
{
    try
    {
        objData = new DBFile();
        DataSet dsCompanies = objData.GetCompaniesList(loggedInUserId);
        if (dsCompanies != null)
        {
            if (dsCompanies.Tables[0].Rows.Count > 0)
            {
                ddlselectcompany.DataSource = dsCompanies;
                ddlselectcompany.DataTextField = "CompanyName";
                ddlselectcompany.DataValueField = "CompanyID";
                ddlselectcompany.DataBind();
            }
        }
    }
    catch (Exception ex)
    {
        lblMsg.Text = ex.Message;
    }
}