I hope you are using twoway binding everywhere ?
<combobox height="40" horizontalalignment="Stretch" verticalalignment="Center">
Name="vormComboBox" Grid.Column="1" Margin="2"
ItemsSource="{Binding ElementName=materiaalComboBox,Path=SelectedItem.Vormen}"
SelectedValuePath="Model.VormId"
ItemTemplate="{StaticResource FotoMateriaalVormTemplate}"
SelectedValue="{Binding Model.FotoVormId,Mode=TwoWay}"
SelectionChanged="vormComboBox_SelectionChanged">
<combobox.itemspanel>
<itemspaneltemplate>
<virtualizingstackpanel />
</itemspaneltemplate>
</combobox.itemspanel>
</combobox>