Odd behaviour in the Solution Explorer of Visual Studio 2012

Question

In this article on MSDN the 'new' functionality of the solution explorer is explained. It includes details on viewing the logical elements of a file.

I have a number of solutions, all of which are 2012 solutions. On some of these, if I click on the arrow next to a .cs file, the node expands to show the name of the class and that, if expanded, shows the names of the methods, etc.

Other solutions, however, doesn't expand the file to show the contained class.

This is a real problem as, when using the modelling features, I have to drag a class from the solution explorer, not a file.

For the life of me I can't work out what is different between one solution and the next.

Has anyone any suggestions about how I can get all my solutions to allow me to expand a file's content in solution explorer to show the logical content?