Textbox textchanged event not firing in vs2010

<asp:TemplateColumn HeaderText="Amount">
    <itemtemplate>
        <asp:TextBox ID="txtAmount" runat="server" CssClass="txAlign" Width="100px" MaxLength="15" AutoPostBack="true"  OnTextChanged="txtAmount_TextChanged"
             Text='<%# DataBinder.Eval(Container.DataItem,"c5") %>'>
    </itemtemplate>

protected void txtAmount_TextChanged(object sender, EventArgs e)
    {       
        double totalval = 0.00;  
        foreach (DataGridItem item in dgBill.Items)
        {
            if (((TextBox)item.FindControl("txtAmount")).Text!= "")
            {
                totalval = Convert.ToDouble(totalval) + Convert.ToDouble(((TextBox)item.FindControl("txtAmount")).Text);
            }                       
        }
        txtTotAmount.Text = totalval.ToString();        
    }