As title suggests, even when I set Hint property of checkbox, tooltip is not displayed when mouse cursor is hovering over the control. There is no problem with the same Hint property over TButton control. Why?
Make sure the TCheckBox.ShowHint or TCheckBox.ParentShowHint property set to True.
In the latter case, make sure the Parent's ShowHint/ParentShowHint property is True, and so on up the Parent hierarchy. If you reach the TForm an everything is enabled, then make sure TApplication.ShowHint property is true, and that there are no OnHint event handlers that could be discarding hint notifications from being dispatched to the TCheckBox.