Visual Studio 2010 with TFS can't edit source code

Once in a while when I open a project in Visual Studio and get latest version of source from TFS and I go to type in a source code file it will not auto check out and will not let me type anything.  The quickest way I have discovered to fix this is to click on another source code tab(or open another source code file if there is only one tab available) then go back to the original source code and you should be able to start typing.