You should also check that you haven't pinned a set of attributes to be displayed for the class. If you have, it will override the DebuggerDisplay attribute specification.

When at a breakpoint, right click on an instance of the class object and add it to the Watch window. If you've pinned elements then clear the pins. Your desired attribute listing should now appear.

It took me quite a while and some luck to figure out why my one class's display didn't work.