I set the side to AEF, the top to A. The lens was set to f/2.8 indoors, lamp light. I pressed the shutter release all the way, and nothing happened. (Well, the camera beeped and flashed near the dark slide icon but I'm using the Polaroid back so it probably doesn't know it's out.) Anyway, I'm guessing it was just too dark and refused to release.
I took off the prism and shot with the WL finder just now, with a manual shutter speed (metered at 1/4 second, shot 1/3). It fired as expected and gave expected results, too.
I need to read up on this prism a little more.
Thanks so much for your replies!