Update panel is not updating more than one time

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ASP.NET

I am using table inside update panel. Each row has textbox and image button.
On clicking on the image button, new row will appear with textbox and image button.

When I click the image button for first time, I getting a new row with Textbox and image.
But it's not working for second time, but event is triggering in code behind.

My aspx code:

<asp:UpdatePanel ID="UpdSystem" runat="server" UpdateMode="Conditional" >
            <ContentTemplate>
                <div style="max-height: 150px; overflow: auto; width: 100%;">
                    <asp:Table ID="tblSystem" runat="server" Height="25px">
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem1" runat="server" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem" runat="server" AlternateText="System" ImageUrl="~/Images/plus1.png"
                                    OnClick="CreateTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem2" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem2" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem3" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem3" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem4" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem4" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem5" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem5" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem6" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem6" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem7" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem7" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem8" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem8" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem9" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem9" runat="server" Visible="false" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                        <asp:TableRow>
                            <asp:TableCell>
                                <asp:TextBox ID="txtSystem10" runat="server" Visible="false" CssClass="input-page"></asp:TextBox>
                            </asp:TableCell>
                            <asp:TableCell>
                                <asp:ImageButton ID="imgSystem10" Visible="false" runat="server" AlternateText="System"
                                    ImageUrl="~/Images/minus.png" OnClick="HideTextBox" />
                            </asp:TableCell>
                        </asp:TableRow>
                    </asp:Table>
                </div>
            </ContentTemplate>
        </asp:UpdatePanel>

Please help. its very urgent. Thanks in advance