WPF Frame Content's binding not updating

Hi

I have this code to update the frame's content with binding.  It initially loads the Page but not on subsequent changes in the Property it's bound to :

<Frame Navigating="DisplayFrame_Navigating" x:Name="DisplayFrame" Grid.Row="1" 
Content="{Binding Source={StaticResource NavControl}, Path=NavPage, Mode=TwoWay, 
UpdateSourceTrigger=PropertyChanged }" />

Any suggestions?  Thanks.