Right-click, navigate to the Filters menu, and select "Show all tags".
Verify that the document was recognized as C++ (maybe you think it is, but it is really an extensionless header file). Quickest way is to look at the Document > Select Mode (...) menu.
Is is really blank, or does it indicate that the file is too large? (one line message at the top of the Defs tool window). You can double-click on that message to force the Defs tool window to update.