How do I ensure that Two Way Binding really is two

问题:

Continuing my experimentation with building wpf user controls. I now have the basic end user customisation features fixed and the navigation control can bind to a data source (an ICollectionView). I have noticed though (despite setting the binding to the view to be Two Way) that when I click the buttons on the navigation control the records displayed in the grid are in sync;

but if I select a row on the Grid the Record x of Records textbox on the navigation control does not get updated to reflect the new selection on the grid.

Obviously I was naieve in thinking that just setting the binding to be two way would have the desired effect. What have I missed?

This is an mvvm scenario if that makes any difference.

Thanks