AUTHOR: Marin Bratanov
DATE POSTED: October 15, 2018
<asp:Placeholder> and <asp:Literal> controls and their content do not get updated by RadAjaxManager.
RadAjaxManager looks for rendered content that it will wrap in an update panel. This means that the controls it AJAX-enables need to render on the page with the ID provided in the AjaxSettings.
Placeholders and LiteralControl instances do not render a wrapping element with their ID and so RadAjaxManager cannot get a proper handle on them. Sometimes this can also be true for user controls, depending on the concrete control.
You can get the same behavior for controls that initially had Visible=false so they did not render.
Wrap the content in an element that will render an HTML element with its ID. For example, an <asp:Panel> or <asp:Label>.
<%--instead of ControlID="PlaceHolder1"--%>