AddingNewItem method not called on datagrid combobox selection change in wpf datagrid

I have a WPF datagrid with first column as a data template column with combobox control. I allow the users to add rows to the datagrid. When the user changes the selection in combobox( which is the first column in the grid) the AddingNewItem method is not called. But when is the user clicks on the next column which is a datatextcolumn, the method is called. How can make the AddingNewItem method to be called on the combobox selection changed event.