I've found the same thing as people have expounded upon. I turned down a show at NCAR after speaking with a pro photographer friend who had shown there 3 times with almost no sales -- some calendars & books, I think. The biggest issue for me is that it's a big space, which means big prints (expensive to do) and then needing to store the prints somewhere after the show. I just keep a rotating gallery in my office, and patients are always eager to see what's new on the walls. I've been at a national juried show where there was a couple: the wife really wanted to buy something & the husband said he wouldn't put anything on the wall that he didn't take.