From the information provided, it seems that you are using AJAX to add the new item to your repeater.
You are also probably using the RadToolTipManager TargetControls collection to specify which items should be tooltipified.
However, it is very likely that your AJAX code does not cause the RadToolTipManager control itself to be updated on the client-side. (even though you might be adding correctly the new item to be tooltipified in its target controls collection on the server).
Since we do not know wich AJAX you use (RadAjax, MS AJAX) we are not sure where exactly things go wrong.
However, in case you are not able to resolve the problem with the provided information, please open a support ticket and send us your project - once we are able to run it locally, we will send you back an updated working version.
the Telerik team