Ribbon Button state not updated when bound to a DelegateCommand

I use your ribbon buttons to perform several actions, all of which are bound using the buttons Command parameter to a DelegateCommand property in my ViewModel. Application logic may enable and disable the canExecute parameter of the DelegateCommand, but the button enable state does not change. I am also firing a PropertyChanged event whenever enable/disable logic changes occur so they can propogate to the View, but this has no effect.

If I hide and then show the ribbion bar, I found that the changes were pushed out to the view properly. Why doesn't onPropertyChange events modify the enable state of the ribbon:button automatically?

At one point in the project, we were using the application button, and everything worked fine (or so I thought). It seems that what was really happening, is that when the app button was clicked on, then the underlying button states were updated.

<ribbon:Button x:Name="saveTestButton" Label="Save Test" KeyTipAccessText="S" Command="{Binding Path=SaveTest}"/>

private DelegateCommand saveTest;
private bool saveTestEnabled = false;
...

public DelegateCommand SaveTest
{
     get
     {
         return saveTest;
     }

     set
     {
         saveTest = value;
         onPropertyChanged("SaveTest");
     }
}       
...

saveTest = new  DelegateCommand(SaveTestAction, canSaveTestAction);

private bool canSaveTestAction()
{
    return saveTestEnabled;
}

private void SaveTestAction()
{
       //Save test stuff...
}

 private void onPropertyChanged(string propertyChanged)
{
      if (PropertyChanged != null)
      {
           PropertyChanged(this,new PropertyChangedEventArgs(propertyChanged));
      }
}