Have you called Hummin Bird and talked to them? Very likely, something is wrong with the unit or the transducer. You should be seeing something. Does the regular sonar give you decent information?
DI is not my favorite, but it is another functional way of looking at structure.
My DI and SI seem to show HARD objects much better that soft stuff like fish or thin brush, but I might need a new transducer myself.
Don House and J.E.Van Natta are two guys that are local and know the Hummin Bird stuff inside and out. Maybe one of them will see this thread and chime in on it.