I semi-recently posted a Show HN that attracted very little attention, received no upvotes or comments, and yet I don't think the system is broken.

The volume of Show HN's is just so high now, thus "standing out" is more difficult. The shownew page has tons of projects and one in every handful receives interest. It seems to be that the audience is there, but they have naturally become more selective.

Of course there remains a huge timing and luck element. Are the people in your demographic browsing shownew at the time you post? Are they even on HN? These things are out of your control.