wpf combobox inside a listview doesnt work on filtering

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C#

WPF

I have a listview in which comboboxs are provided with search option.
Here there is a country combobox on the listview header, by its selection change the state combobox on the listview want to get filtered. Same want to happen in state combobox selection change, city want to get filtered.
I cant access the combobox by its name because it is inside the listview, it is accessed by its object.
Here filtering is occurring successfully but the filtered combobox doesn't shows its popup. By pressing the Down arrow keys we can see the filtered data. but its popup cant see. Any solutions??