This happens to me pretty often. For example, right now I have the debugger stopped at a breakpoint in a method . . . and it isn\'t displaying any variable values at all.
I figured out why it is not working in XCode 4.6 - all of the variables in my object, self, were declared in the .m file instead of the .h. When I moved one of them back to the .h file, it showed up in the debugger. Sounds like a bug with XCode in that it cannot "see" variables declared in the implementation file.