Need help with the WPF Sort with a ViewModel

Question

I've created a nice treeview with a viewmodel and it is wrapped around the data models with view models, however, I am having an increasing issue with getting a sort and filter to work with the view model that I already have in place.

Can someone please take a quick look at my source code and let me know where the issue I am having?

Source code is attached.  VS2010 with .Net 4.0

Thank you.