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.
If you are using the @property feature of Objective-C 2.0 the debugger does not display those variables unless they are backed by explicit ivars in your Class interface. This is slated to be fixed in Xcode 4 as I understand it.