Vista - Object Binding Query

Using Object Binding how can I get changes in the class to update the UI?

In a win form VB2005 app I've got a simple class and created the binding
source using the wizard and dragged the text boxes for the fields from the
data source window onto the form.
The form load event creates an instance of the class and stets some values
for the class and then sets the bindingsource datasource to the class
instance. The UI shows the class property values properly and changes in the
UI alters the values in the class properties so the binding would appear to
be set up, but if the class values are subsequently changed in code the UI
does not alter. How do you make the changes in the class property values
show in the UI?

Many thanks

Julian