ComboBox selected value not binding for second time wpf mvvm

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

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