I am not completely sure why you do not see the tooltip at all - what I can assume is that it actually appears for a while on an expected place due to being a child of the rotator item. This being said, if this is the problem, you should set RenderInPageRoot="true"
/the default value is false/ and this will ensure that the tooltip will be added to the form and respectively correctly displayed.
A sample demo which explains this setting and uses exactly the scenario you describe is available below:
the Telerik team