I assume that you have used the following online demo as a start point for your implementation:
If so and if you also have an update panel as in the demo, please make sure that the RadToolTipManager is also wrapped in the update panel.
Let us know wether this helps and in case it does not, please provide more details about your setup and the exact problem , e.g does the tooltip not show at all, does it show at first but then disappears, etc. and we will do our best to help.
the Telerik team