Keynote: Can't see presenter notes when using external monitor

I use an external monitor to display slides in Keynote. Connected via HDMI port. Laptop display set to "main" and external set to "extended." According to Apple, when I start a presentation in fullscreen, I should see it on the external (this does happen), and the laptop should show the presenter display (this doesn't; the laptop just goes black.)

If I press x, the displays switch as instructions say they will: the slides are on the laptop and the external goes black. But it's still black.

The best I can do for presentations is to mirror the displays, and then at least I see the slide on my laptop. I know this isn't how it's supposed to work. What am I missing?