I have moved your thread to the RadNotification forum and I would advise that you post your questions in the most relevant place, as this will facilitate getting a better response.
As for the issue - most likely, the problem is related to the AJAX setup. To update a server property and have that change reflect on the client, the control in question must be part of the partial page rendering. Thus, if in the problematic user control, the notification is not updated on the client, it will come up empty.
Note that AJAX settings may not necessarily be in the user control, it is possible that they are applied to the parent elements of the user control.
Apart from that, one idea I can offer is using the Show("new text") override of the method that does not require that you update the notification with AJAX. You can see it in action here: http://demos.telerik.com/aspnet-ajax/notification/examples/servershowwithnewtext/defaultcs.aspx. Note that, to change the Title, you would still need to update the control with AJAX. Thus, you can consider using a static title and icons, if this fits your needs.
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