The behavior you describe most probably comes from the fact that the RadComboBox as all RadControls is complex HTML and it is formed by different standard HTML elements. Thus you should specify the exact element which should be tooltipified because otherwise you will actually tooltipify another element which I assume is the case with you.
Please, share your code - both the combo and the tooltip manager and how you have tooltipified the combo here and I will examine it and let you know what is the exact reason for the issue. Provide also more details about the exact desired result and I will do my best to provide a solution for you.
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