wpf Combobox Itemssource not binding within Listbox Data Template

<StackPanel>
  <ListBox Width="400" Name="lstFiles" ItemsSource="{Binding}">
    <ListBox.ItemTemplate>
      <DataTemplate>
        <Grid Name="dataGrid">
          <Grid.RowDefinitions>
            <RowDefinition/>
          </Grid.RowDefinitions>
          <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
          </Grid.ColumnDefinitions>
          <TextBox Name="dragFileName"
                   Margin="5,0,0,0"
                   Text="{Binding fileName, UpdateSourceTrigger=Default}" />
          <ComboBox Grid.Row="0"
                    Grid.Column="1"
                    Margin="5,0,0,0"
                    Name="cboDragDocType"
                    ItemsSource="{Binding dragDocType, UpdateSourceTrigger=LostFocus, Mode=TwoWay}"  
                    Text="{Binding dragDocTypeText}"
                    IsEditable="True"
                    IsReadOnly="True" />
          <Button Name="dragDelBtn"
                  Grid.Column="2"
                  Height="20">X</Button>
        </Grid>
      </DataTemplate>
    </ListBox.ItemTemplate>
  </ListBox>
</StackPanel>

Data is loading in comboBox by using ObservableCollection property. In Initialize method i do for loop method to get multiple items. Problem scenario: Select item from the dropdown1 --> then Select item from the dropdown2 -->Again click dropdown1, Now dropdown1 itemssource is empty.