Are you using the ContentTemplate of the RadWindow? Have you modified the z-index the RadWindow has? Is your RadWIndow maximized? I am asking this because it is possible that the dropdown shows, but the z-index values cause it to stay behind the RadWIndow in case you are using the ContentTemplate. More information about the default z-index values is available in this help article
and it also shows how you can override it so that the combobox has higher z-index than the RadWindow. By default the RadWindow will be below the combo's dropdown, which is why I am asking if you have modified these values. In case you have your RadWIndow maximized you can set its ShowOnTopWhenMaximized property to false, because its default value (true) causes the maximized RadWIndow to increase its z-index value to 100 000.
the Telerik team
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