Hi Brian Mains,
"Initiates a load-on-demand callback request with the specified text, causing the ItemsRequested server event to fire or a request to be sent to a WebService. The second bool parameter instructs the combobox to append the new items (true) or clear items (false)."
So, in your case you can set the second parameter to false to make sure that new Items are loaded.
the Telerik team