RTC VS Plugin - Why is 'Annotate' Disabled?

I'm using the RTC plugin for Visual Studio 2013 (Version: 6.0 Build: I6.0_20150519-0433). For some files in our solution, the Annotate context menu item is disabled (though all other items, like Show History and Check-In, are enabled). The files are checked-in, shared (according to Jazz Properties) and have no local changes. Why might this be happening?

EDIT: Additional info: The common factor appears to be files that, when double-clicked in the Solution Explorer, open a designer instead of the source file directly. 'Simple' classes have Annotate enabled properly, whereas WinForms UserControls and forms do not (though the double-clicked file is, in fact, a *.cs file at heart). The *.Designer.cs files do not have this issue, and also do not open a designer by default when activated in the Solution Explorer. *.xsd files are the same way (DataSets).

For all of these files, even when viewing them in text form (opening them in Visual Studio and editing the underlying text, rather than loading the Designer), Annotate is disabled. Setting the default opening strategy via Open With... --> Set Default has no effect.

The context menu for one of these files is below (there's no difference in the Jazz section of the context menu between the context menu when right-clicking on the file in the Solution Explorer and when right-clicking on the file's text when loaded in CSharp Editor rather than the form designer).

(Gotta use an imgur link; not enough reputation to attach an image)
http://imgur.com/a/GR5NQ