ViewModel在提交时未正确更新Model

I am trying to use a strongly-typed ViewModel and find that when I replace the Model class with a ViewModel, changes are not submitted correctly from the Edit template. Whereas just using the straight Model class in the View, edits happen successfully. My model class is MaterialDefinition and the ViewModel class is MaterialDefinitionViewModel as shown below. I've updated the edit template correctly to reference the ViewModel but as I say editing just does not work. I'm using VS2013 and MVC4. Any ideas, someone?I am trying to use a strongly-typed ViewModel a