Gridview textbox textchanged event not firing sometimes only in ASP.NET C#

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C#

SQL-Server-2008R2

ASP.NET4

<asp:UpdatePanel runat="server" ID="UpId" UpdateMode="Conditional" ChildrenAsTriggers="true">

<asp:TemplateField HeaderText="Closing Reading" ControlStyle-Width="120px">
                                                                                        <ItemTemplate>
                                                                                            <asp:TextBox ID="txt_closing" runat="server" Text='<%#Eval("closeReading") %>' EnableViewState="true" 
                                                                                                AutoPostBack="True" OnTextChanged="txt_closing_TextChanged"></asp:TextBox>
                                                                                        </ItemTemplate>
                                                                                        <ControlStyle Width="120px" />
                                                                                    </asp:TemplateField>

</ContentTemplate>                                                                               
                                                                            </asp:UpdatePanel>

What I have tried:

more than 100 value bind gridview, textbox textChanged event correctly run but sometimes only event not firing .....???