ComboBox selected value not binding for second time wpf mvvm

Question

Hi am using wpf mvvm am able to bind combobox value for the first time but when i close the window and click again am not able to get the combobox value binded.  

 

This is my view Model code 

 

private async void LoadData()

{

//getting all volumeunits (kgs, cc  in dropdown)

LstUnit = ProHelper.GetVolumeUnits();

} 

and xaml code

 

 

<controls:ProTextBlock Text="Unit :" Grid.Row="6" Grid.Column="4" Margin="2" />

<controls:ProComboBox Grid.Row="6" Grid.Column="6" Height="22" DisplayMemberPath="Description" TabIndex="1"

SelectedValue="{Binding JobRegister.BookingAssociate.VolumeUnit , UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"

SelectedValuePath="Description" ItemsSource="{Binding LstUnit, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">

 Waiting for the reply as soon as posiible .

 

 

 

Thanks In Advance