Why do Image Buttons do not work?

[NMDUC]

Hi,
I have a list of Image Buttons like this.

<div id="nav">
<ul>
<li><asp:ImageButton ImageUrl="~/images/profile.jpg" runat="server" ID="ibtnProfile" AlternateText="Profile" Enabled='<%# (HttpContext.Current.User.Identity.Name != null) && (HttpContext.Current.User.Identity.Name != "") %>' OnClick="ibtnProfile_Click" /></li><!--
--><li><asp:ImageButton ImageUrl="~/images/reviews.jpg" runat="server" ID="ibtnReviews" AlternateText="Reviews" OnClick="ibtnReviews_Click"/></li><!--
--><li><asp:ImageButton ImageUrl="~/images/myblog.jpg" runat="server" ID="ibtnMyBlog" AlternateText="My Blog" OnClick="ibtnMyBlog_Click"/></li><!--
--><li><asp:ImageButton ImageUrl="~/images/friends.jpg" runat="server" ID="ibtnFriends" AlternateText="Friends" OnClick="ibtnFriends_Click"/></li><!--
--><li><asp:ImageButton ImageUrl="~/images/bookmarks.jpg" runat="server" ID="ibtnBookmark" AlternateText="Bookmark" OnClick="ibtnBookmark_Click"/></li><!--
-->
</ul>
</div>

And the click events are on code behind.
protected void ibtnReviews_Click(object sender, ImageClickEventArgs e)
{
....
}
........

When I click on the image buttons, it will call the suitable event. However, sometimes it does not work- nothing happened when I click on the image button.

Please help me. Thank in advance.
Du