Asp.net button onclick event is not firing serverside code
<p>This has been discussed too many times already, but none of those threads helped me.</p>

<p><strong>Below is .aspx code.</strong></p>

<pre><code>&lt;div id="Panel_login" runat="server" class="panel-default"&gt;
     &lt;asp:UpdatePanel ID="UpdatePanel_login"  UpdateMode="Conditional" runat="server"&gt;
        &lt;ContentTemplate&gt;
             &lt;asp:RequiredFieldValidator ID="txtb_loginEmailRequiredFieldValidator" ControlToValidate="txtb_loginEmail" runat="server" ErrorMessage="Email can't be Empty"&gt;&lt;/asp:RequiredFieldValidator&gt;
             &lt;asp:TextBox ID="txtb_loginEmail" CssClass="form-control" placeholder="Your Email" runat="server" TextMode="Email" Width="400px"&gt;&lt;/asp:TextBox&gt;
             &lt;asp:RequiredFieldValidator ID="txtb_loginPasswordRequiredFieldValidator" ControlToValidate="txtb_loginPassword" runat="server" ErrorMessage="Password can't be Empty"&gt;&lt;/asp:RequiredFieldValidator&gt;                                        
             &lt;asp:TextBox ID="txtb_loginPassword" CssClass="form-control" TextMode="Password" placeholder="Password" runat="server" Width="400px"&gt;&lt;/asp:TextBox&gt;
             &lt;asp:Button ID="btn_userlogin" runat="server" OnClick="loginUser" Text="Login" /&gt;                             
        &lt;/ContentTemplate&gt;
    &lt;/asp:UpdatePanel&gt;
&lt;/div&gt;
</code></pre>

<p><strong>Below is my .cs file code.</strong></p>

<pre><code> protected void loginUser(object sender, EventArgs e)
 {
     Response.Redirect("www.google.com");
     log.Debug("login_user is called");    
 }
</code></pre>

<p>On click of button, it should fire event, but it is not happening.</p>

<p>I have tried adding triggers to Update Panel for this button click, still didnt work.</p>

<p><strong>It only gets fired when another button on same page is fired previously.</strong></p>

<p>Can someone suggests what I am missing here.</p>

<p>Thanks for help.</p>